Spiral Knights Catch-All: F2P MMO action-rpg from Three Rings
It's open, it's free, it's a quick download, it's made by the guys behind Puzzle Pirates and it's a fun co-op dungeon romp with light puzzle elements. What are you waiting for? Taste it!
Previewers seem to call it a mix of Zelda, Guild Wars, Secret of Mana and Bomberman.