Although for a very different use case, Glimpse (@abramdawson shared this earlier here - http://www.producthunt.co/posts/94, cc its CEO, @elan_miller) also uses Instagram photos as a vehicle for discovery. Skedadel uses existing UGC to discover restaurants/clubs/etc. and Glimpse uses it to discover people.
There's soooo much UGC created (and much more to come as wearable tech becomes ubiquitous) that will lead to more interesting use cases like this.
I'm a big fan of "invisible" apps - this one is great because it uses data people already create (instagram photos) and ties to usefulness (see what's nearby, and tag your favorites). Skedadel feels more friendly than foursquare because it's a personal list rather than just a leaderboard.
Just saw Zofari (http://www.zofari.com/), a "pandora for places" as TC describes it. They offer an in-browser demo of a fully functional product on the homepage. Kind of neat but slow. Might be better to show a quick animated GIF of the user experience and instruct users to download from the App Store.
Was just going to post this but Ryan told me it was already up. Love Skedadel!