Dead Space is a very cool and innovative game, though it can be a bit scary if you're sitting home alone at night playing it. The first time I played it, I was doing just that, and it was fairly creepy
Mirror's Edge is also a very fun and unique gaming experience. I own all four of the most recently added titles, and I think those two are by far the best (and probably the two that every gamer should at least try once).
Army of Two is fun if you have somebody to play co-op with, but the single player isn't that bad either. Games like Borderlands and Left 4 Dead have definitely set new standards for co-op play, so I wouldn't put it at the very top of my co-op must-haves list or anything. There's a sequel coming out in 2010.
I have mixed feelings on Lord of the Rings: Conquest. If you played any of the previous Lord of the Ring games published by EA, you may be somewhat disappointed with the combat system. I'm not a huge fan of the Battlefront franchise either, and this uses the same basic framework. However, they made pretty decent use of the licensing and the ability to play through both good and evil campaigns was a fun addition.
Three of the deals offered by Microsoft have expired and returned to their retail prices (Batman, Beatles Rock Band, Band Hero). Bakugan was still $29.99 the last time I checked. Updated the original post accordingly.