Open source stuff is tops. I would argue that there is not an application out there in open source that won’t fill the gap for something that you pay for. I have a link to a whole list of applications that fit this criteria in the Web Links. Note that this list is Windows specific but covers everything that you could think of and then some. But I have found the mother load!
Games are great, especially ones that you do not have to pay for. Just skimming through this list again I would argue that there would not be a game type or genre that is not represented. One my personal favourites and that of my mates is BZFlag. A first person tank game… the tanks can jump too! Great game and great fun.
While these games might not win up against the likes of commercially produced games (yeah the ones that you pay $100 bucks a pop for) but they are pretty good considering that most developers do it for the love of it. In addition they contain the elements that are necessary to make a game great: fun, playability, functional, multiplayer, free! In some cases they are excellent, not because they are “pretty” to look at but because of their excellent non tangible elements, like being fun. They often do it better as well. Just don’t write open source software off because it is free, the days of free being crap are long gone.
Open source games are often really customisable. If you are into setting a server up, then this is even more so. James set up an excellent BZ server, alas now dead because he became a Mac convert. But it was total fun and exciting to play in and tweaked for the handful of us that used it. One final aspect that I would point out is that the community is very active, always developing and assisting each other. So the help is there if you need it as well. To them a bug is just another challenge!
So if you are interested head on over to the Wiki for the A-Z of Open Source Games!