Bugherd is designed for this but it is still at an early stage -- too unstable for our production use as of Aug 2011. (Very impressive, though.)
Diigo is not designed for bug tracking. It kind of works but I can't really integrate with our bug-fixing workflow.
Apart from the obvious screenshot->save image->upload image to a bug tracker, are there ways to allow our users to easily point-and-click and input bugs?