Google URL Shortener has just launched its own website. The feature worth mentioning is that you can view statistics on all URLs you have shortened. The statistics include referres, traffic details and visitor profiles.
Google URL Shortner is:
Stable: Nearly 100% uptime
Secure: Automatic spam detection based on the same type of filtering technology we use in Gmail
Speedy: Doubled speed
There are unofficial extensions available for Chrome: goo.gl URL Shortener and Firefox: goo.gl lite.