We have big threads devoted to the more popular games like Batman, Shadow Complex and Left 4 Dead 2, so here's one for the games not many people are talking about. 
I MAD3 A GAEM WITH ZOMBIES 1N IT!!!1
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802585502a6/
Indie game selling for a whopping 80 MS points ($1). This game is worth the price of entry just for the music, which has an original song that's funnier and catchier than Portal's end theme IMO. The Smash TV meets Geometry Wars gameplay with trippy special effects don't hurt either--this is a lot more polished than the typical Indie game fare. Aside from the lack of Xbox Live integration (although it does have 4 player local play which I haven't tried), this twin-stick shooter is very much worth your time if you want to see an original take on the genre.
Avatar Drop:
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802585502da/
Another Indie game selling for 80 MS points ($1). What can I say, I'm a sucker for simplicity. Your avatar along with three others is dropped onto a map containing rings and giant balloons...and you fall, complete with ragdoll physics in place. You guide your avatar by hitting right or left, getting points for going through the rings. The person with the most points wins, not who gets to the bottom first. A nice diversion that probably should have been free when avatars rolled out, but the asking price was right.
Defense Grid:
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802584108cb/
Xbox Live Arcade game selling for 800 MS points ($10). I'm a tower defense noob, having played only Pixeljunk Monsters which to this day remains one of my most played PS3 games. This one had me from the beginning--I was hooked after 2 or 3 levels. While the formula stays close to the tower defense formula, the sheer amount of eye candy is staggering when there's lots of enemies on the screen. A big bonus is that there are a TON of gameplay modes (two difficulty levels, play with a fixed number of turrets, play with only basic turrets, speed modes, etc.). Unlike other tower defense games, this one appears to have a lot of longevity. I can't believe I played for 4 hours last night...
What lesser-known games are you playing? I'm always on the lookout for sleeper titles, particularly on the Indie and XBLA fronts, especially considering my most played game on the 360 is an Arcade game (Carcassonne).

I MAD3 A GAEM WITH ZOMBIES 1N IT!!!1
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802585502a6/
Indie game selling for a whopping 80 MS points ($1). This game is worth the price of entry just for the music, which has an original song that's funnier and catchier than Portal's end theme IMO. The Smash TV meets Geometry Wars gameplay with trippy special effects don't hurt either--this is a lot more polished than the typical Indie game fare. Aside from the lack of Xbox Live integration (although it does have 4 player local play which I haven't tried), this twin-stick shooter is very much worth your time if you want to see an original take on the genre.
Avatar Drop:
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802585502da/
Another Indie game selling for 80 MS points ($1). What can I say, I'm a sucker for simplicity. Your avatar along with three others is dropped onto a map containing rings and giant balloons...and you fall, complete with ragdoll physics in place. You guide your avatar by hitting right or left, getting points for going through the rings. The person with the most points wins, not who gets to the bottom first. A nice diversion that probably should have been free when avatars rolled out, but the asking price was right.
Defense Grid:
http://marketplace.xbox.com/en-US/games/media/66acd000-77fe-1000-9115-d802584108cb/
Xbox Live Arcade game selling for 800 MS points ($10). I'm a tower defense noob, having played only Pixeljunk Monsters which to this day remains one of my most played PS3 games. This one had me from the beginning--I was hooked after 2 or 3 levels. While the formula stays close to the tower defense formula, the sheer amount of eye candy is staggering when there's lots of enemies on the screen. A big bonus is that there are a TON of gameplay modes (two difficulty levels, play with a fixed number of turrets, play with only basic turrets, speed modes, etc.). Unlike other tower defense games, this one appears to have a lot of longevity. I can't believe I played for 4 hours last night...
What lesser-known games are you playing? I'm always on the lookout for sleeper titles, particularly on the Indie and XBLA fronts, especially considering my most played game on the 360 is an Arcade game (Carcassonne).