Currently I'm juggling between Team Fortress 2, which just received a huge arse free update, Red Dead Redemption single player, and Starcraft 2 beta, which also just re-opened up to the public very recently.
Chances are, since the beta is limited, I'm basically going to be only playing the SC2 beta and skipping out on TF2 (the stuff will still be there, and a gazillion people will still be only playing Engineers)until its over. I'm almost finished RDR and I'll play it once in a while in between.
Of these three games, I think all of them are amazing. TF2 needs no explanation - Fantastic fun, in addition to free updates that keep the game fun long after release. SC2 beta is also fun for me, as I like the gameplay so far, in addition to seeing what amazing creations map makers have made. As for Red Dead Redemtpion, I cannot play multiplayer but the single player has been amazing so far. I love how they nailed everything in all categories. The setting, story, visuals, audio, gameplay, and fun factor are all up there.