A Game of Tennis - indie game. It's Pong.
Bastion- a ARPG like Secret of Mana. It's most excellent.
Bionic Commando Rearmed - an overhaul of the old NES classic.
Borderlands - first person looter, best played with friends
Braid - a physics based puzzle platformer. One of the games that first convinced me to get a 360, along with
Castle Crashers - A side-scrolling beat-em up. You probably already have this.
Castlevania: Symphony of the NIght - Classic sidescroller that co-birthed the term "Metroidvania" for exploration based platforming.
Dragon Age: Origins: I bought this the last time I got a points card and played through all the beginning stories. Good if you like BioWare RPGs.
Ecco the Dolphin- Another classic I picked up the other day, that I had played back in the day, but had a hankering to play again so I picked it up. 2D dolphin swimming light puzzle solving. Awesome music/atmosphere still. And now it has achievements!
Fallout 3/NV - 2 of my favorite games this gen. 1st person RPG ala Elder Scrolls but sci-fi post apocalyptia goodness instead.
Guardian Heroes - another sidescrolling beat 'em up like Castle Crashers.
Limbo - Physics based puzzle platformer with a black/white aesthetic. It drips with bleakness.
Marvel vs Capcom 2 - classic Capcom fighter I bust this out once in a while still. I know there's 3 and origins but I don't have them, so I can't speak for them.
Puzzle Quest - gem matching puzzle game with rpg elements.
Scott Pilgrim vs the World - Sidescrolling beat 'em up like Castle Crashers, but it takes more notes from River City Ransom on the NES than anything else.
Shadow Complex - The next gen equivalent of Super Metroid/Castlevania Symphony of the Night
Secret of Monkey Island - HD remake of a beautiful and funny point and click adventure game.
The Cave - I loved the Cave. Some critics derided some aspect of its level design, but I still thought it was worth $15.
The Walking Dead - TellTale's narrative driven take on The Walking Dead.
Those are all the games I've spent money on so far on XBLA. I think they were all worth it. Your Mileage May Very. If anyone has any suggestions for me as to what to get next, I'd welcome them. Oh, and I also have Battleblock Theater. Which is, of course, great. Oh, and I also have D&D Daggerdale, but I wouldn't recommend it...