Pendlay rows (not cable rows or regular barbell rows) and deads should add nearly all the width you could want with appropriate diet. But from your comments about working low/mid back but not upper it sounds like what you're actually looking for is something that involves the traps to a high degree. Rows and deads work them to some extent, of course, but nothing beats power/jump shrugs for trap development.
Madcow has a link to a bunch of good descriptions of dynamic shrugs on his site. They're a fun and very effective movement, can be done for low or high reps, and, if that wasn't enough, have nice carryover to the top portion of the deadlift.