Imfy.us is a URL shortening service for people who want to ensure the users clicking through their shortened URLs are actually people and not web-indexing search engine crawlers.
Submit a URL to Imfy.us and the resultant shortened URL will direct to a landing page at Imfy.us. On the landing page you'll find a preview of the site and a CAPTCHA system—the source of Imfy's anti-crawler abilities.
Want to take the CAPTCHA idea one step further and actually secure your shortened URL with a password and not just against crawlers? Check out previously reviewed dwarfURL and HideLinks for two URL shortening services that allow you to add a password requirement to your shortened URL.
Have a favorite URL shortener? Know of a URL shortener with a trick or two up its sleeve? Let's hear about it in the comments.