Unfortunately the frameworks used by most modern PC games are almost all DirectX based, which is a microsoft technology. There are emulators that have come a long way in bridging the gap (Wine tries to run most windows apps, and http://transgaming.com/
has produced the Cedega software which offers support for PC gaming at a monthly charge and seems to work pretty well).
Also, Valve recently announced that they are
moving forward with a linux client, but to what end I don't really know yet.
The reality, however, is that until we as a people start telling game manufacturers that we want them to use open source or cross-platform frameworks (such as OpenGL) to build their engines Linux gaming is just not going to be up to par.
edit: cedega can be found here: http://gametreelinux.com/