Yesterday's new Xbox Live Arcade additions were Metal Slug XX and Aqua.
Of the two, Metal Slug XX is more expensive (1200 Microsoft Points). This is a 2D shooter with several multi-path levels to wade through. There a couple of new characters to mix things up, plus a Combat School with over 70 missions to tackle. The Metal Slug XX Xbox page has a bit more information.
Aqua costs 800 Microsoft Points. This is Steampunk naval warfare; you're a little ship that must chug around blasting other bigger ships. There's multiplayer. The Aqua Xbox page offers a few screenshots.