The demo of Insomniac's Fuse, formerly Insomniac's Overstrike, melds with the Xbox Live Marketplace and PlayStation Network today. It's already available on XBLM (all 1.45GB of it), while PSN gets it later today.
The demo lets you play a portion of the four-player shooter's campaign either offline with AI partners, or online. If you go online you can play solo with the option to invite friends, or with friends able to join you at any time. You can also play co-op via matchmaking.