Guess what? GNU/Linux is great for playing games!
It is an operating system that has a totally fair multitasking system to divide processing power smoothly to various processes that are running so that nothing gets all the processor's power while some other task gets it in fits and spurts.
As an example; for those that like to dual-box(running multiple instances of a game at the same time on one machine) it is normally much smoother to do than it is on windows. This is straight-up, not fan-boy stuff.
It was designed from the ground up for smooth network access at all times so no matter what the computer is doing, or what other things you have running while playing the game, your game client will not boot you due to dropped packets, nor result in network lag. My ping time in WoW was normally in the teens being a west-coast player on an east-coast server, and I can have a lot of things going on in the background.
I can: transcode a video from a native format to MP4 for uploading to YouTube, fileshare Several Linux ISO disks via BitTorrent, run Rythmbox(a music player) drawing the music from the local NAS, have my E-mail client up polling for email, plus I have a full LAMP webserver running on this machine, and while in Dalaran in World of Warcraft I still get 50+ frames of video with no network lag. Granted, I have a quad-core 64bit proc and a smoking video card, on a high-end motherboard...but this kind of thing would be difficult for any machine to pull off if the operating system is not properly allocating the resources so everything operates smoothly.
Also, unlike the windows operating system; no matter how many applications you install, or how many updates you apply, the computer will not slow down over time. This may be fixed in more recent releases of windows, but I am told it was still a problem as of vista.
Games that are written to run natively on GNU/Linux will perform the best of course, but as of now very few major releases have native clients for any distribution of GNU/Linux. The Ubuntu distribution is changing that fairly rapidly, but as of now the vast majority of games that people know from advertising and such are written for Windows...but many of these games will also work quite on GNU/Linux because of the use of an application called the WINE project. WINE is, as with many GNU/Linux projects, a self-recursive anagram that means {W}INE {I}s {N}ot an {E}mulator.
WINE is best called an 'application compatibility layer'. It places itself in-between a game or any other Windows application, and GNU/Linux. It intercepts various system calls from the application, and performs the equivalent action inside of GNU/Linux. The Windows program running this way has no idea it is not running in a Windows environment. A full list is available on http://www.wine.org, under the AppDB tab at the top right-hand corner of the main page.
There are 3 levels of compatibility that the WINE project recognizes regarding native windows applications running on WINE. These follow a sort of Olympics theme:
Platinum: Applications which install and run flawlessly on an out-of-the-box Wine installation
Gold: Applications that work flawlessly with some special configuration
Silver: Applications with minor issues that do not affect typical usage
As of today; 12-29-09, the 10 top games according to user voting in WineHQ.org are:
Platinum Rated Game List
1) Star Craft: Brood Wars
2) Guild Wars
3) Photoshop CS2
4) Counter Strike (Retail and Steam versions)
5) Supreme Commander
6) Half-Life 2
7) Watchtower Library
8) Silkroad Online
9) Spore
10) Day of Defeat (Steam version)
Gold Rated Game List
1) Eve Online: Dominion
2) Steam Online Gaming Client (all versions)
3) The Sims 3
4) Bio Shock
5) Runes of Magic
6) Homeworld 2
7) System Shock 2
8) Planetscape: Torment
9) Soldat
10) S.T.A.L.K.E.R. Shadow of Chernobyl
Silver Rated Game List
1) Final Fantasy XI Online
2) Warhammer Online Live
3) World of Warcraft
4) Fallout 3
5) Team Fortress 2 (Steam version)
6) Aion: The Tower of Eternity (versions after 1.5.0.6)
7) Left 4 Dead (Full version from Steam)
8) Command & Conquer 3: Tiberium Wars
9) Warcraft 3: The Frozen Throne
10) Call of Duty 4: Modern Warfare
Please keep in mind that regardless of the overall rating of these titles, if it is your intention to play one of these titles on GNU/Linux utilizing WINE then you should visit the WineHQ.org and look up the title. There may be comments from other players that have some tips on configuring the game client to play best under WINE.