HOO-AH! America's Army: True Soldiers reporting for duty! In demo form. On The Land Of The Free's Xbox Live Marketplace. 535MB.
As the description says: "Created by Soldiers, Developed by Gamers, Tested by Heroes". Vomited on by me. And yet it's not available here, sadly, so you will need to mount an unauthorised invasion of the US Marketplace to acquire it.
If you do, you will discover that you can team up with up to 15 other players to tackle enemies entrenched in a coastal village for a "dynamic co-op mission", or you can do an eight-on-eight "Squad Engagement" match.
You'll get to use all the real stuff the US Army uses in the field today, like the Blue Force Tracker and the Raven UAV but not the UN. Anyway, that's enough xenophobia for one morning.