I know that there are several Firefox plugins that are invaluable for development. What plugins exist that are useful for system administration, monitoring, and the like? What plugins make your day-to-day job as a system or network administrator easier?
ShowIP - allows me to see quickly the IP address of the server where a particular website is hosted. Assists in managing my many clients websites.
DNSCache - quickly disable/re-enable Firefox's builtin DNS Cache, particularly good if your also manipulating the site's DNS at the time
ScreenGrab - particularly good at capturing that error and sending to the developers.
Foxyproxy - I cant stand getting attacked by others when they walk over with their stats and say "Hm this Serverfault.com you wasted 5 minutes of our time on it."
Tamper Data this is handy when you have to examine HTTP headers. This may be necessary if you have virtual hosts in a hosting environment. We also insert a field in the header to identify web machines in some of our web farms to identify problem hosts.
Delicious Bookmarks to sync my bookmarks across machines.
LastPass. So I don't have to remember the loads of accounts I use (and can generate very strong passwords for each of your accounts), both personal and at work.
If you're using amazon ec2; elasticfox
Nagios Checker is pretty nice.
ReloadEvery - so I can get SO refresh automagically while working!
AdBlockPlus - because so many sites have ads that I don't care about.
Charles Proxy with Firefox plugin is much better than Firebug network statistics.
Fireftp for when you don't want to install an ftp client
In addition to others already mentioned, I find SQLite Manager invaluable.
You can't work without something to give you rhythm