If you're into using Instagram to upload and share photos taken with your iPhone, you'll probably love Pic a Fight, a site created in a the spirit of FaceMash (of recent The Social Network fame) that pits Instagram images against each other. The site was created using the app's API and pulls photos only from users who give permission for their use.
If you're not an Instagram user, the good news is you can still cast your vote on the site. And if you are an Instagram user, signing into the site gives you a profile page with all of your images on one page, showing you the number of wins and losses each photo receives and the number of points associated with each. Users can also view the best-scoring images and up their game to match.