If you want to have fun, PC gaming is the way to go, just need to know where to look and what genre you like.
Something simple, free, and MMO is
Phosphor. FPS gaming on PC I'm sure is the most popular genre because of the power and control method of PC's.
Another genre is racing. The popular free racing game right now is Trackmania Nations, which is basically a high-speed stunt racing game with loops and jumps. As with 99% (estimated) of all retail games, there's also demos, the only ones I have now are Need For Speed, the latest ones are probably best for testing how far your PC can take the graphics.
Flight sims are also good for anyone with a joystick. You can go with combat, racing, or regular flight. Warbirds is one of my all-time favorite combat sims, Plane Crazy for racing, and MS Flight Simulator for flying.
The Battlefield series could be used for nearly every genre except MMO or anything non-3D, especially with the help of mods (need the full version for that).