A co-worker mentioned this gem to me today: www.realmofthemadgod.com
It's essentially multiplayer (100 users?) gauntlet with progression, itemization and end goals in an open-world. It's quite fun!
From the wiki:
Realm of the Mad God pits every player against Oryx and his innumerable monster horde. Killing as many of his creatures as possible to summon the Mad God himself for a duel is the main objective of the game. However, there's more to see and do.
Band together with other players to defeat 25,000 monsters, Skull Shrines, Cube Gods, Pentaracts and Oryx himself will show up to duke it out. All players on the server will be transported to his lair and he will appear on your mini-map as a purple dot in the middle of the map no matter how far away you are. It will take a team to defeat him, as he continually spawns his own demons to shield himself from your attacks. Oryx can take 20,000 damage before he winks out of existence and pledges to return after another 25,000 monsters are killed. He generally leaves behind some nice loot for his vanquishers.
Collect the Best Equipment
As you level up, you'll find plenty of equipment that your class may or may not be able to equip. Find other players via chat to trade loot with until you have the best equipment for your character as described in the Equipment section. The maximum level is 20, but equipment is not limited based on player level.
Farm the Gods
In addition to dropping good equipment, the gods also drop potions that can permanently increase your stats. Many players have taken to farming the gods for these potions, but you have to be careful... kill off too many and they won't be able to reproduce!
On the character selection screen it's cool to see unloackable classes.
Warning - PermaDeath!