Nobody is suggesting free ones..? Hmf. Well here are some free/libre co-op RPGs that fit the bill:
Stendhal: A retro graphics simple MMORPG. Not many people play (~20 online) so you mostly have the world to yourself. (my SO and I played this a lot, lots of fun to explore & do dungeons together!) Still actively improved & developed and the people who play tend to be friendly. Big world with lots of dungeons and items and monsters.
Egoboo: I'm surprised no one has mentioned Egoboo it's free & open source, 4 player coop on one comp with controllers, silly & fun theme, very similar to Diablo / roguelike gameplay.
If you enjoyed Rogue, Nethack, Moria, Angband and the other Rogue-like games back in the old days, there's a modern MoRPG game you might like to try, Stendhal. Multicolor graphics, stereo sound, many maps and more. So complex it can take five years to make the top level. A great way to waste time during breaks or at lunch.