Poladroid is a little app that takes any photo and turns it into one looking like an old style polaroid. You can even watch it develop and shake it to develop faster. ;) It will take an arbitrary crop of your image so if you want a specific crop, it's better to crop it before conversion. I'm usually not a big fan of image alteration but this is cool and I might take a few photos specifically for polaroiding.

Here are a few examples: