Google maps + flickr
I have only just seen this. These guys have hacked google maps and are using it to show flickr photo locations in. This process is known as GeoTagging and this is the cleverest version I have seen; and I might add, is apparently ok with google (so far). I also read whilst snooping that one of the google maps engineers hinted that google may well realease an API for google maps so people would no longer need to hack it. I look forward to this.