Flickrball is a six-degrees-of-separation scavenger-hunt game, using Flickr thumbnails and tags for clues. It's been working since October of 2005, and as such is probably the very first Web 2.0 game. To play:
Note the starting and ending tags listed at the top of the page. The ending tag is your goal. What's on the page right now is a set of 49 thumbnails, all of which have your starting tag. Mouse around until you find a thumbnail you think might have a tag that leads you closer to your goal, and click it to zoom in.
When you zoom in on a thumbnail, you'll see its tags. If you see a likely-looking tag, click it for a new set of thumbnails. If you get stuck, you can back up by clicking any of the tags you've already chosen.
For reasons of tradition, the default version of Flickrball starts with "kevin" and ends with "bacon." If you hate this idea, feel free to substitute your own start and end tags like cat to dog, soup to nuts, alpha to omega, or whatever.