There are plenty of great Xbox Live games in the Marketplace but there are way more indie games to be found and here are two that rise to the top.
Impossible Shoota is actually one of the top games in the Marketplace, sitting at number nine for free games and it has had over 13,000 downloads since its release just under a month ago. It's a classic ship bullet game with simple controls, great effects and some rocking sounds. A new update was just released which includes leaderboards and some bug fixes. Cost = free, so there is zero reason not to try this game if you want some quick arcade style fun.
The second is less known, which is a shame because it's pretty amazing. Called Decimation X2 by Xona Games, it's basically Space Invaders on steroids. With huge explosion effects, rapid fire game play and fantastic music, it's tons of fun. Plus, the company has Decimation X and the brand new Decimation X3 for the Xbox 360--those latter games are in the top five in Japan and we can see why after playing X2. The cost is $0.99 which we think is worth it and there is a try-before-you-buy. (Correction: Xona Games is based in Canada & X2 doesn't have bosses, just X3; they're also working on a small update to increase the finger bar size for easier usage)