Started life based on janl's io http://github.com/janl/io Other than some very 
basic structual similarities, this is a complete rewrite.

To install:
couchapp push url-shortener http://127.0.0.1:5984/urls

Create a short url:

curl -X POST http://127.0.0.1:5984/urls/_design/shortener/_update/shorten?target=http://zombo.com/

You should see:

  Shortened to ~439tppb

If you try to shorten the same url again, you should see a document conflict, 
since a shortened url already exists for that domain:
{"error":"conflict","reason":"Document update conflict."}

Get the redirect:

http://127.0.0.1:5984/urls/_design/shortener/_show/long/~439tppb

Example config running behind an Nginx reverse proxy:

location /~ {
    rewrite ^(.*)$ /urls/_design/shortener/_show/long$1 break;    
    proxy_pass http://127.0.0.1:5984;
}

For a list of shortened urls in the database see:
http://127.0.0.1:5984/urls/_design/shortener/_list/index/shorts

The main page for the app is:
http://127.0.0.1:5984/urls/_design/shortener/index.html

The shorten method uses a basic crc-32 checksum on a base64 encoded url, then 
returns a base36 string of the checksum. I ran a test on ~700,000 urls I 
downloaded from a blacklist site (probably the only good use for censorship). 
There is approximately 1 conflict per 10,000 urls. Most of the collisions 
occured with ranges of similar ip only urls.