I like the annotation feature of Flickr, where one can add notes to pictures and they show up as you mouse over them. What I don't like is that feature is only available with Flash.
Something like that should be possible using CSS and some JavaScript.
So I sat down one night and hacked this out. It's not quite perfect (you have to click in the boxes to see the notes, and one of the notes runs too long) but as a “proof-of-concept” it works.
I just need to work on this a bit more.