I have a dual G5 tower that I've decommissioned here at home for desktop usage. What free server alternatives could I install on it?
You didn't really mention what you were intending to do with the system as a server. Video streaming? File serving?
What clients are you using? Macs have some services that work well with other Macs, so you could do some basic services right from a regular 10.x OS X installation and have some interoperability with other Macs as clients.
If you're serving just files, a basic web server, etc. or have a reason not to use OS X anymore you can install Debian or YellowDog Linux.
This question really depends on what you're familiar with for configuring and administrating, and what you're aiming to serve with the system.
I have a decommissioned G5 Mac and I configured it as a family computer for my son (4 years old) because it's relatively simple for him to use and with enough memory he can use it to watch DVD's and play basic flash games. His sister (17) steals the keyboard for typing papers in OpenOffice/NeoOffice and some online research tasks. Either way I could set the system to also be a web server and print server with the basic OS X installed on it if I wanted to.
I've had good experiences with Debian on PPC hardware (or anywhere else, for that matter). FreeBSD should also run on that box; running a BSD is great fun too.
I'd stay away from running Mac OS X as a server unless you need something that doesn't run on a proper UNIX (like FileMaker Server); the filesystem is horrendous and overall performance is rather poor, in my experience.