Have been searching around now and then to try and find a decent paintball game for the PC. There are many that are like Half Life 1 and doom 2 mods, but they just don't give you a decent paintball-like feeling and experience. Only until the other day I found one that is the best built and designed paintball game I ahve ever come across. Its call Splat Magazine's Renegade Paintball. The link to download the demo is here:
http://www.fileshack.com/file.x/7893/Splat+Magazine+Renegade+Paintball+Demo/
Thats the demo and it should get you hooked quick enough. To buy it its just $19.95 or $19.99, can't remember which it was exactly, but I bought it and Cartboy, Ferret, and Myself have played this for the past couple days and it is some great fun. Here is a link to some screenshots of the game:
http://www.globalstarsoftware.com/renegadepaintball/preloader.htm
I have had lots of fun with it so far and thought I would spread the wealth. Kinda helps satisfy the need to play some paintball when you can't get enough people to play or whatever the reason. Anyway enjoy.