I'm searching for a link shortening service that can be installed on a shared host just like WordPress. It doesn't need to be very featured, I just need it to shorten URLs, be free, and can be installed on a shared host.
YOURLS (PHP, MIT).
However, you don't need any fancy software for a personal URL shortener. You can make one PHP file in less than 10 LOC: write an array, and a HTTP-redirect if your argument matches any key in the array.
Polr (PHP, GPL2)