I do. I run Ut2004, Doom3, Quake3 + mods and several other games. Then there are literally thousands of other games out there, some good, lots of bad, mostly smaller indie type things.
For everything else I have a playstation 2 and a gamecube.
The biggest issue is the hardware. Right now the ideal gaming rig in my opinion is going to be Nvidia video card, Via motherboard (from a good quality manufacturer), AMD proccessor. Stay away from weirdness like on-board raid0/1 stuff (linux can do a better job thru it's own software raid and then you don't need special controllers. (keep all the drives as masters though, no slave drives in a raid array)), the new PCI-X stuff is touchy, and SATA controllers seem to be in contention right now, but that will clear up in the next few months. The best hardware are the ones whose manufacturers contribute to free drivers thru documentation and code, but with vid cards it can't be avoided and Nvidia is the clear choice.
Other then that you have a few big name popular games that run natively in Linux, and for the rest you have to depend on a Win32 api/DirectX-compatablility layer for linux called
Cedega (formally WineX). Note that it isn't emulation it's a clone of the Win32 and DirectX programming stuff for Linux, it's not nearly 100% though and some games work better then others. Also be carefull because Cedega costs a little bit to obtain and they list games that have native versions in Linux which do not need their product to work. (native games are much perferable to games thru Cedega)
Also Wine is the original prodject that started the Win32 api for Linux, but Cedega incorporates many closed source stuff like cdrom copy/install protection that Wine will never support. Halflife 1 is a good example of a game that can be run thru Wine for free. Half-life 2 is a good example of a game you need Cedega to play in Linux.
Right now the state of gaming for Linux is that there are plenty of things to have fun with if your a Linux user, but Windows is still a preferable gaming-only platform. (pretty much everything else Linux is as good as Windows, or surpasses windows in, IMHO. The main that that Windows has over Linux is gaming, some propriatory windows-only applications, and lots of people are familar with Windows)
some links:
Linux gamers.net
Happy Penguin
LinuxGames
And there are dozens of other websites and such. It's a nice community as far as gaming goes...