In recent memory, I was pretty damned impressed with Asura’s Wrath – don’t know about the final product, but the demo made a strong case for it. And The Darkness II is a neat little vignette cut from the rest of the game that shows you murdering while “quad-wielding” is just as fun as akimbo.
Staying in tune with longer, meatier demos, check out Catherine. Puzzle gaming is not everyone’s strong suit but the game’s very Japanese infusion of anime storytelling kept me wanting to play more than the gameplay itself.
On the XBL Arcade front, I strongly recommend checking out either of the Zombie Apocalypse titles. I prefer the original since the sequel forces you to suck on unfunny gamer stereotypes while stealing its mechanics from PSN’s Dead Nation. The original reminds me of Hunter: The Reckoning simplified down to small arenas where you have to machinegun hordes upon hordes of the living dead.
Also, if you can find indie games under the deluge of fucking advertisements the new update cursed us with, download Beat Hazard. Imagine Geometry Wars if the ammo you fire, the enemies that swarm you, and the difficulty of levels all depended and varied based on the music you have saved to your harddrive. If you’re sensitive to trippy, pulsing lights, ignore this paragraph.
Also, spend some time with Insanely Twisted Shadow Planet, you won’t regret it.