Sega's PSU, like Square Enix's Final Fantasy XI, is one of the few highly successful console-centric MMOs to date. It has a complete single-player campaign and a persistent online world which can be explored by groups of up to six players.
If you're playing PSU on the XBox 360, you can download the expansion pack for 1,600 points. If you're a PC or PS2 player, you'll have to buy the retail game disc. Japanese players have had access to the expansion since September, and Europe will have to wait until January to get its hands on it.