Defense Grid: A couple of dollars and the most polished and balanced Tower Defence game you'll ever play, looks nice to boot.
Shadowgrounds Pack: These are a top down version of what Doom 3 should have been. Intense action, excellent moody lighting effects and a decent enough story/universe. You get both titles for about $5, they're both decent length and support upto 4 player offline co-op on the same machine.
The "Shooter" games: A combination of Diablo and Smash TV, they're 99p each or a few quid for the pack, if you're picky go for Alien Shooter 2. Loads of loot and loads of shooting with a huge selection of guns and an awesome visual style.
Trials 2: The predecessor of the most popular Arcade game on the 360 this year, sure it needs no introduction, hard as nails.
Machinarium, Braid, World of Goo and Audiosurf are all utter non brainers as well, just buy them, don't even bother thinking about it.
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