Hyatt from Excel Saga- Very good to have around when the repo guys come to take your stuff. All the blood would be hard to get out of the carpet though.
Zoro- Keep things clean. Two words: Santoryu mop. Also helpful with the ugly nerds next door. He gets rid of them and comes back home to be sexy!
Naru from Love Hina- Need help with homework? No problem with Naru around!
Sanji- I'm allergic to cigarette smoke, so not so great. I can't cook worth shit though.
Not really from an anime, but
Hermione Granger- Punching bag! (I'm gonna get in trouble for that one…)
Ororon from Akuma no Ororon- Also smokes, but at least he goes outside. Good for keeping demons and fallen angels out of the house. Also can sumon sweet black flames at will, because, you know, he's satan.
EDIT: I should add Hughes from FMA. He'd be an awesome roommate if he wasn't married.... Or dead.