In an attempt to make the game more immediately appealing, the Condemned demo is a tweaked version of the game's first level and features more enemies and more stuff to hit enemies with. This is well worth a download, but be warned that you may end up having to buy the game when you're done with the demo.
Second up we have Quake 4, a direct port of the PC version which hasn't made the transition all that smoothly. The demo highlights the biggest issue with the game: its poor and inconsistent frame rate. You get to play through a couple of short sections from the start of the game and test your aim in the game's online multiplayer mode.
As has been the case with previous demos, these are free to download and play on your Xbox 360.