YOURLS: Your Own URL Shortener. Whistle - personal URL shortener. Whistle is an open source, algorithmically reversible, personal URL shortener.
There is an instance of Whistle running at Note: if you're looking for an open source, database-based, generic (any) URL shortener, see other projects like ur1.ca (note: that's U R ONE dot C A), yourls.org, etc. feel free to suggest others.