Metal Slug XX, Aqua on XBLA
SNK or swim.
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.