Flickr's image annotation is neat, but relies on Flash for presentation. I'm not inherently opposed, but I've thrown a few students on the shoals trying to recreate it. They've mostly foundered on the rampup to learning Flash.
Gina Trapani and Stuart Langridge have come up with imagemap and DHTML combinations that have a similar effect. I'm guessing they're both a little easier on the learning curve, and much easier to generate programatically.