ooh, a few good ones not yet mentioned - time for some thread necromancy!
Goblins - Life Through Their Eyes, an affectionate parody of the rpg genre, with goblins as protagonists, pretty long running (since 2005), art and plot improve massively from those first few issues!
Girl Genius, a steampunky alternate history tale about… someone named Agatha Heterodyne… (that’s about how much can be told without spoilers )
Freak Angels, by Warren Ellis (of Hellblazer and Transmetropolitan fame) and Paul Duffield (first thing I’ve seen of him, but appears to be an established cartoonist as well).
“23 years ago, twelve strange children were born in England at exactly the same moment. 6 years ago, the world ended. This is the story of what happened next.”
Features a flooded London, steam-powered helicopters and miniguns, mind-controlling mutant powers and a fuckton of foulmouthed british dialogue.
Finished at 144 episodes (of 6 pages each)
also, +1 for both Order of the Stick, and Romantically Apocalyptic… and yeah, Oglaf is simply hilarious too!