Microsoft just announced the new XNA Creators club feature where community created games can make their way on to xbox live arcade. They have a couple sample games that you can download now. I really recommend "The Dishwasher"
This is a limited time demo over the next 17 days so check them out now before they disappear.
Instructions:
1) Download XNA Creators Club Game Launcher (Marketplace/Game Store/All Games/XNA Creators Club). Ignore the others.
(Note:
If you use "XNA Game Launcher" or "XNA Game Studio Connect", you'll be
told you need a Creators Club membership linked to your profile.)
2) Browse to My Games (Games/Games Library/My Games) and twist to XNA Creators Club.
3) Press Y to Download Games
4) Choose a game to download like any other game.