imgur...there are no ads anywhere.
As much as I love imgur, that is false. There ARE minor ads on the non-hotlinking pages. Much less of a pain in the ass than imageshack and Photobucket for sure, but they are there.
I've never used it before, but I've heard decently good things about
Minus. The file size limit is 2GB, but you have to sign up for a free account if you want them to last for more than 30 days.