Even though it's currently just a demo, BeatBuddy has stellar potential, especially since just the demo got me hooked and left the song stuck in my head for the next few days. The most fun I've had with a rhythm game since Taiko Drum Master for PS2.
If you always wanted the modern FPS Star Wars always deserved, play Republic Commando IMMEDIATELY! Released in the days when Halo1 and Halo 2 were king, this shooter married the best elements of those two with Star Wars. It proved that a Star Wars FPS with plenty of attention to detail and all the fun of Halo most definitely would work. Multiplayer needs some slight fine tuning, but is still loads of fun, especially if your buddies only have the first Xbox and are tired of just Halo 2; this game will feel refreshing and plenty of fun. Also, campaign is a blast, and leaves you begging for more.
There is almost no way to have more fun on a PC than to play TF2. With 9 classes to choose from, and a thriving and passionate multiplayer community, one wonders what separates TF2 from the Call of Dutys, Halos, and else. The one word is BALANCE. Almost everything can be countered, and every advantage has its drawbacks. Sentry pinning your team down? Spys can sneak in and take it down. Rockets flying everywhere? Pyros can reflect them. All the problems and stalemate-instigators that plague other class and non-class based shooters TF2 solves with delicately-balanced play that actually makes teamwork a must and carefully designed maps that can be played for hundreds of hours and still present new strategies evey play. Oh, and it has cool hats n' stuff.