Some odd things have been coming up in the Steam database recently, like a few older games that never received PC ports as well as some that are just unexpected. For me, the biggest surprises were:
Halo's 1-3
KOF XIII, 2002 and 98
Brutal Legend
Some things have been happening since then and now, Brutal Legend is on the Steam store:
http://store.steampowered.com/app/22..._7_7_204_150_3
MS also announced that Halo 2's multiplayer support would be terminated on PC shortly before this was discovered. Now there's been a u-turn and they've said that Halo 2 will continue support until June 2013 while "other support options are investigated".
Think I'd pick up the Halo trilogy. Think I'd have a lot more fun with a mouse and keyboard. not only that, but Halo 3's forge mode with a mouse would be a godsend.