http://hipstaroid.com/
Why?
Inspired by a lot of the retro photos I see being posted from iPhones I decided to enable retro photos for existing Facebook photos. Uses the Facebook photo tags to tag the 'polaroid' with sharpie.
API?
Yes, there is an API if you want to use this for other things. I'm planning on making a non-Facebook interface. Anyway for the API use the following:
http://hipstaroid.com/hipstaroid?img=<url>&caption=<your caption>
1) The image on your front page has some pretty bad compression artifacts, which is kind of a turn off.
2) It would be cool to allow color effects, like sepia or black and white.
3) Simple design is great. You may want to add a little more to your design, though, to make this feel like a more solid app.