Most people have already heard of Ding! and Looking For Group. There's others out there, and I'm just too lazy to mention them all, except to highlight this one:
This is a snippet of a comic called The Scout Report, which chronicles the misadventures of a Night Elf hunter named Scout, who has about the moral compass of your average warlock. It's done using screen shots which work incredibly well, and best of all it's nothing short of hilarious.