If you haven't tried Zeno Clash for PC, then check XBLA for the latest version. Atlus' remake of ACE's first-person brawler has a few upgrades over the original and is dubbed Ultimate Edition.
Priced at 1200 Microsoft Points, Zeno Clash: Ultimate Edition comes with all new Tower and Pit challenges, which you can tackle with a friend via splitscreen co-op or online. The campaign is still single-player, but new weapons and attack moves can be used in all modes. A time challenge mode is also included, just under a different name: Zeno Rush.
The entire package is definitely worth $15, and you'll probably keep going back to the character gallery. Zenozoik is home to more freaks than you can count.