SEGA released a demo of their new game, Universe at War: Earth Assault for XboX 360, and it's available through XboX Live network.
The game actually came out in March, but it seems that sales weren't going so well so SEGA executives decided on releasing the demo. That would enable potential buyers to try before they buy, and it's only fair because no one likes blind buys.
We came across a couple of unfavorable reviews, and that might also be one of the reasons why SEGA decided on a demo. The demo is 547MB and gives you an overview of space battlegrounds that the full game offers.