I still haven't cracked open my Left 4 Dead (because I know once I do it will consume me for hours), but I did download a couple things for the XBox in the last couple days.
I downloaded the Roy Orbison pack for Rock Band 2. Haven't played it yet, but figure I will either in the next couple of days or on my usual Saturday night online game with friends.
I also downloaded the 1910 expansion for the online Ticket To Ride game, which was 400 gamer points (about $5) and it was totally worth it. It adds on three new game modes, with a Globetrotter card (most routes connected) to replace or augment the Longest Route rule, and there's also a Big Cities variant where all the destinations connect through one of the "big" cities. These are minor tweaks, not unlike the Carcassonne King & Baron expansion, but like that expansion, they add a lot to the game without making the game play overly complex.
In short, definitely recommended.