Trick.ly is a free url shortening service with an innovative feature to password protect the shortened urls. Whenever your password protected shortened url will be accessed, trick.ly will ask visitor to enter the password, the user will only be directed to the original url on entering the correct password.
You can also add a clue with password to the shortened url, which will enable your friends or the desired people to guess the password.
Other than password you can also protect the url with a Riddle or a Question. [via]