I generally don't participate in threads which quickly deteriorate into people recommending Windows-only programs, but I'm going to make an exception. (I'm even going to start the linking!)
The only reason I even maintain a Windows install anymore is for two games: Subspace and FUO (Free Ultima Online).
For Subspace, all you need to do is go to
http://www.subspacehq.com
Short description: A 2d overhead team-based space ship shooter. It's the world's oldest running MMOG and the community is for all intents and purposes fantastic. There are a lot of dumb servers, but if you're looking for good quality fun, play the 17th Parallel. 17th takes some skill though. Most newbies go to Trench Wars first.
FUO is a community of player run Ultima Online servers. It's very well organized and there are dozens of quality servers to play on. To play you'll need a UO client. A quick ISO download from a P2P community or a quick $20 will land you the client. From that point forward, everything else is free. Just go to
http://www.uogateway.com and download the gateway client to connect to free servers. I recommend the UOGamers and Metropolis servers.
And as a man extremely picky when it comes to gaming communities, I can assure you both games have high quality communities. Those games are both so good IMHO that it's still worth maintaining a Windows partition.
Happy gaming.