i know there are at least two companies that provide software that does this, sorry - i can't remember the names of either of them.
i know that they weren't freeware and from what i experienced myself the annotation method wasn't 100% perfect. reloading pages would sometimes move the annotations around a little bit.
this was all a few years back so whoever it is that made the program(s) may well have got it all working correctly by now.
personally, i'd love to see an annotation tool done as a 'coding snack' - i know i'd use it if it worked properly.