I've never had much interest in the content of Xbox Live Arcade, but Shadow Complex completely redefines what gamers can expect from downloadable content. At first glance, it's just a two-dimensional shooter (like Metroid) with some very nice graphics. But the game is superbly designed, with a variety of weapons and items, inventive use of three dimensions in an 2-D game, and a surprisingly well-written story. Not to give too much away, but you play as an average joe with an above average amount of training in combat, who finds himself trapped inside the massive headquarters of a shadowy group determined to overthrow the United States government. By the end of the game, you've become an unstoppable bad-ass in power armor.
There's a number of different ways that the player can beat the main story, and also a challenge mode where players can test their skills getting through various obstacle courses. And the entire thing costs only $15.
Batman: Arkham Asylum
A licensed game based upon a comic book that doesn't suck! That alone would probably make Arkham Asylum worth checking out, but it isn't just a great comic book game, it's an outstanding game period. The story is straightforward: Batman captures the Joker and brings him back to Arkham Asylum, but Joker's pals have already seized control of the prison and are in the process of using it as a base to launch an all-out attack on Gotham City.
The gameplay is quite varied. You can get into fistfights with dozens of Joker's goons, using a combo system that seems facile but actually allows for some remarkably complex combat. Or you can stalk your enemies as a silent predator, leaving them dangling from gargoyles or knocking them out with your batarangs. There's also an investigation mode that's simple but helps break up the monotony of the fights. And there are a ton of collectibles, as well as clues left by the Riddler.
And the game looks gorgeous. If you're a fan of Batman, this game is a must get. Even if you couldn't care less about Batman, it's worth checking out.