What features would you offer with a URL shortener?

I recently launched a url shortener as a 'bit of fun' more than anything. It took less than 4 hours of my time and $100 of my money to build and bring to market.

I'm now wondering what features I should add to make it more attractive and interesting to use. Interested to hear Hacker News' users thoughts!

UPDATE: thought I should add that I already have a REST API and that I'm working on making the entire database of key/value pairs available. Wondering what to do after that.

Thanks, Ben