Lost Crypts is just the co-op dungeon crawler you're looking for. Played entirely in your browser, this retro-styled RPG puts you in floor after floor of dimly lit corridors stocked with enemies, spawners, gold and treasure. Along with up to three other players, it's your job to mine each area for all it's worth, then head down to do it again. Shockingly fun, and the co-op is simple but inclusive enough to work.