Url-Shortener is a project mainly written in PHP, it's free.
Basic URL shortener with comment frame!
Had to have a try at this, since they were popping up everywhere.
Uses a completely random string(5 char in length of 52 possilble chars). Which is a enormous amount of possible strings, but still should check against the database. So, should be improved by using a hash of some sort.
Oh well.