Don't you just hate it when other people build something you thought of? WikiMapia is a wiki combined with Google Maps, allowing people to annotate a map geographically and add notes.
I have to say, the implementation is pretty slick. The map and navigation itself are of good quality, and intuitive. And the annotation capability is reasonably well-done... you can do non-rectangular regions, and it's pretty smart at deciding what level of detail to show you (per your zoom level).
It's a bit sluggish, but then it's trying to do a fair bit. One of the best things I like about it is that as you reposition and zoom, the Url changes immediately. That's very important; if you've tried using Google Maps, you'll discover that it's very difficult to email someone a Url to what you're looking at.
I do have to say that the actual "traditional" wiki part (text and photos) is a bit light; it doesn't feel comfortable and display is only supported over top of the map, which I also find obtrusive. It might be better if they instead allowed you to view the clicked item in the _search pane on the left side of the browser window (in MSIE and FireFox, maybe others too). This would make it scrollable, resizeable, and allow you to continue your map exploration without closing pop-ups.