The Phantasy Star Universe (PSU) demo is on Xbox Live right now. This is the open beta of the upcoming 360 MMO, and will only be playable until October 18th at Midnight.
No word yet on whether the progress your characters make in the demo will translate to the full game when it is released.
PSU weighs in at 940 MB so plan accordingly.
Lastly, it's worth noting that this game will cost $9.99 per month when the full game is released.