I believe every system administrator is used to open source by now. From Apache to Firefox or Linux, everyone uses it at least a little bit.
However, most open source developers are not good in marketing, so I know that there are hundreds of very good tools out there that very few people know.
To fill this gap, share your favorite open source tool that you use in your day-to-day work.
*I will post mine in the comments.
I love PuTTY !
The PuTTY executables and source code are distributed under the MIT licence, which is similar in effect to the BSD licence. (This licence is Open Source certified and complies with the Debian Free Software Guidelines.)
Notepad++ lightweight, has excellent support for different formats, my main text editing tool in windows.
Synergy lets you easily share a single mouse and keyboard between multiple computers with different operating systems without special hardware. It's intended for users with multiple computers on their desk since each system uses its own display.
It is also platform independent.
Nobody mentioned screen yet?
UnxUtils: This is a port of various gnu shell utilities based on msvcrt.dll so it understands native windows paths - i.e. you don't need to map to a /cygdrive path. This is a key advantage over Cygwin if you have to interact with native windows commands or homebrew CL utilities.
Strings: is a very good way to scrounge through files for items of text. Many, many uses.
Flex: Really designed for writing lexical analysers, with a little bodge artistry and a C compiler it can be used as an uber-grep. I don't use it all that often but it can come in surprisingly handy in that role.
Fetchmail and Procmail: Core of my email system for well over a decade, since I had dial-up internet connectivity. If it ain't broke ...
rdesktop: an open source RDP (terminal services) client that works surprisingly well.
PythonWin:, particularly as packaged in Activestate Python. Python on Windows works a lot better than you might think. When used with COM Makepy it's really good for scripting COM APIs.
Wget: an exceedingly useful FTP/HTTP downloading tool.
Leafnode: if you still read any of the newsgroups that still have decent active traffic this is quite a good way to do it. Again, a bit of legacy from my dialup days but it still gets used on occasion.
Abiword and Gnumeric: full featured wordprocessing and spreadsheet software that's far leaner and meaner than OpenOffice.
Xfig: Visio type diagramming tool with an odd user interface. Once you get used to the paradigm it's much easier on my poor old mouse hand than a modern direct maniulation interface. Worth a mention for the ergonomics.
Tcl/Tk: Overshadowed by Perl and Python, Tcl is very easy to embed C code into - it was designed specifically for embedding. Surprisingly useful nonetheless, and the Tk toolkit is very easy to whip up a GUI with. Modern versions support theming so your applications no longer have to look like Motif.
Ghostscript: One of the great unsung heroes of the open-source world. A free postscript interpreter with a whole ecosystem of derived items - PS and PDF viewers, PDF creation tools, printer RIPs and all sorts of Postscript conversion tools. Perhaps most widely used outside open-source circles (if not actively credited) in its role in the back-end of PDFCreator
That's just a sampling of the obscure stuff without mentioning Vim, LaTeX, Firefox, python, gcc, gtk & qt and the Berkeley TCP stack - to name but a few.
My favourite open source tool is rsync.
I use it almost every day and it is still not as famous as it should be:-)
No one mentioned git.
It is not as well known as cvs or svn but I think it will be one day.
7-zip--a file archiver with the high compression ratio. The program supports 7z, ZIP, CAB, RAR, ARJ, LZH, CHM, GZIP, BZIP2, Z, TAR, CPIO, ISO, MSI, WIM, NSIS, RPM and DEB formats.
Vim/gVim - an editor practically no one's heard of!
Wireshark = Network Protocol analyzer.
Kismet = A powerful wireless sniffer.
Tcpdump = The classic sniffer for network monitoring and data acquisition, I use it regularly.
Pound = The Pound program is a reverse proxy, load balancer and HTTPS front-end for Web server(s).
Trac = Project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
Request Tracker = A free web and email-based bug tracking and trouble ticketing system. Features list, documentation screen shots, and download.
Vmstat = The command vmstat reports information about processes, memory, paging, block IO, traps, and cpu activity.
Iptraf = The iptraf command is interactive colorful IP LAN monitor. It is an ncurses-based IP LAN monitor that generates various network statistics including TCP info, UDP counts, ICMP and OSPF information, Ethernet load info, node stats, IP checksum errors, and others.
mc = Visual shell for Unix-like systems.
Postfixadmin = Postfix Admin is a web based interface used to manage mailboxes, virtual domains and aliases. It also features support for vacation/out-of-the-office messages.
pwgen - Automatic Password generation.
Linuxconf = Linuxconf comes with Mandrake Linux and Red Hat Linux, but is also available for most modern Linux distributions. You've probably encountered this tool before if you use one of these distributions, either as the whole package or in one of its modular components. Multiple interfaces for Linuxconf have been available for years, but now we're up to four: GUI, Web, command-line and ncurses.
Webmin = Webmin comes with, and was recently acquired by, Caldera Linux. This tool is not only available for most modern Linux distributions, it also runs on most major flavors of UNIX and is available in around twenty languages (though some modules are not available in all of the languages). As you might guess, Webmin is purely a web-based application and a heavily modular one at that.
OpenVPN = SSL/TLS based user-space VPN. Supports Linux, Solaris, OpenBSD, FreeBSD, NetBSD, Mac OS X, and Windows 2000/XP.
KeePass is a free open source password manager, which helps you to manage your passwords in a secure way. You can put all your passwords in one database, which is locked with one master key and/or a key file.
Screen. It's the most useful tool ever made. Master it and you can be as unto a god, a creature in all places at once.
I use many that I couldn't work without but that I don't consider "not very famous" (openssh, openvnp, apache, rsync, ...). Two very useful little utilities that many may not have heard of sprint to mind:
top with a few useful extra features as well as the prettyBoth can generally be found in standard repositories (they are both in Debian Etch and above) and are relatively painless to compile if your distro doesn't have them.
Edit: another excellent tool that isn't very well known in my experience:
Nagios--Comprehensive IT infrastructure monitoring ensures you can resolve problems before they affect critical business processes....
dstat - imagine vmstat, iostat, top, ps, as well as apache, mysql, etc. all able to output metrics on the same line at the same interval. cross-referencing app-level metrics with system-level metrics is huge.
siege - better than any other URL hammering tool out there
squid - layer 7 routing and caching, quick and easy
maatkit - MySQL is not the same without it
MySQL Proxy - the example lua scripts are enough to make MySQL snooping painless
I have been finding that many people don't yet know about Process Hacker. It's on par with Sysinternals' Process Explorer.
Edit in response to Greg's comment:
Sorry for the delay in responding... It also has 2 tabs that show services and TCP/UDP connection info which I think is really nice. You can get the same info in the services tab in Process Eplorer when sorted by tree view, but then you lose the ability sort within the services list.
In the security side, I will recommend
Both are well known in the security community, but not very much outside of it.
WinSCP, although I'm not completely sure that's Open Source. If not, it's a toss up between Mailcleaner and HylaFAX.
I have to say Squid. I dont think its all that popular, at least not in the Windows world. We use it for many different things: content filter and port blocker included.
My favorite app is the Window Maker, a very lean and fast Linux window manager (similar to KDE, Gnome, etc).
It is not very famous, but available for most distros (on Ubuntu, do apt-get install wmaker).
Small desktop wiki that works on Linux as well as Windows and OSX.
I use it to keep my tasks organized as well as to document things as I go before putting them into the company wiki.
Can you consider a webserver as a tool? If so, nginx has my vote. Else, I would vote for ttcp for throughput testing.
I also find that not a lot of people use xargs. For example, here's a good one I just figured out: how to shred files that have spaces in the filename using find, xargs and shred.
find . -type f -print0 | xargs -0 shred -u -v
I'm a big fan of Filelight. I never knew determining data usage on my hard drive could be so easy or look so pretty.
Ack because it's better than grep:
ack is a tool like grep, aimed at programmers with large trees of heterogeneous source code.
ack is written purely in Perl, and takes advantage of the power of Perl's regular expressions.
I'm also becoming a huge fan of Pandoc:
Pandoc is a Haskell library for converting from one markup format to another, and a command-line tool that uses this library. It can read markdown and (subsets of) reStructuredText, HTML, and LaTeX, and it can write markdown, reStructuredText, HTML, LaTeX, ConTeXt, PDF, RTF, DocBook XML, OpenDocument XML, ODT, GNU Texinfo, MediaWiki markup, groff man pages, and S5 HTML slide shows.
Sprinkle (http://github.com/crafterm/sprinkle/tree/master) - a software provisioning tool.
It is easier than Puppet or Chef, but very powerful. And it is Ruby based, recipes are Ruby scripts.
GraphicsMagick - Lesser known alternative to ImageMagick, built as a single executable and offers more regular updates.
mod_evasive - Allows Apache to take evasive action when under brute force or Denial of Service attacks.
WikidPad - Excellent personal wiki for storing just about any kind of information.
NcFTP - Command line ftp client and FTP library (LibNcFTP).
cURL - Lesser known alternative to wget (also available as a library - LibcURL).
PdfCreator - Allows you to print documents to PDF on Windows.
DeVeDe - Lesser known alternative to DVDStyler. I like it better because it gives you greater control over disk layout, and automagically creates a menu from the layout.
InfraRecorder - Open Source GUI for cdrtools on Windows.
KiTTY - Lesser known alternative to PuTTY, can be carried on a USB memory stick.
Strawberry Perl - Lesser known alternative to ActivePerl for Windows, comes with a C compiler (MinGW), and has a portable version.