Nice, always wanted to set up one of these for myself, but never found a good example. I'm wondering if you would like to tell us (me), what you used to set it up?
site - http://jjbshort.appspot.com/
source - https://github.com/joeblau/jjbshort
edit: I realize the URL's aren't short... the code just needs a short domain :)
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So most likely a Ruby app, and probably not Rails as no asset pipeline has been used.