Twisted is one, and it's there partly because a lot of code relies upon it, not because I like it. I also use a few niche libraries which haven't been bumped and there's a few libraries like mechanize too - not so niche yet still not bumped.

Honestly, if there were some big incentive at this point I would go through the hassle of upgrading, but there isn't. The gains on python 3 seem relatively minor and incremental.