This may well be the "first ever cooperative MMO bullet hell shooter," but I pray to the mad god that it isn't the last. Realm of the Mad God is a massively multiplayer action RPG with all of the quest text and story stripped away, leaving only a lean, arcade style blaster in its wake.
In the beginning, you're just a wizard with a weak wand, but as you hoover up experience for each kill, you'll level, grab new weapons and unlock new classes. There are a few roguelike genes caught up in Realm of the Mad God's weird, tangled DNA. If you die, your character stays dead forever and all his items are lost to the void. It's so fast paced it's hard not to just start over again instantly on death, making this very, very moreish.
It's been free to play for a while on the Realm of the Mad God site, but its arrival as a free to play game on Steam gives us a good excuse to recommend it. Its Steam debut means its scatological collection of worlds will be packed full of low level wizards rushing around shooting chickens. 85 players at a time can jump into Realm of the Mad God's instanced zones, which results in some spectacular, chaotic co-op monster slaying. Give it a go, you almost certainly won't regret it, until your clicking finger falls off.