You can grab Black Ops for $19.99, Black Ops II for $49.99, Modern Warfare 3 for $29.99, and finally, World at War (my personal favorite), Call of Duty 2, 3, Modern Warfare, and Modern Warfare 2 all for $9.99.
Basically, anything from Modern Warfare on still has a decent size community on Xbox Live, so you if you've never played a Call of Duty game and want to try it out for the first time, you really can't go wrong with $9.99. Just take your pick of setting and see what the series is all about this weekend.
Also, don't forget the perpetual sale!
Xbox Live Ultimate Game Sale: Day 4 [Major Nelson]