I'm sure it's been said already, but: Borderlands and (more recently) Borederlands 2. The DLC is basically an expansion. 4 in total so far, all new, full campaigns, and 2 fully different characters with their own skills and abilities. 2 DLCs that give you 11 more character levels a PIECE (Plus a new mode that's harder for said levels, and new guns, and even a new place to do stuff for high levels). And... some stupid costumes, okay, they've made a few stumbles, but at least the costumes are cheap.
DLC Done right. ...except for the costumes. Seriously. Stop that.