Realm of Darkness while not much to look but at is a seemingly very popular site with tons of unique content. The site specializes in SoundBoards and Prank calls.
They have a huge selection of both celebrity prank calls, as well as regular prank calls. They have a habit of calling persons and recording there voices, and then using those voices to call other person, over and over again.
If you are ok with the profanity (as most have more than a little) then this site is awesome. We check it our pretty regularly, and find the humor entertaining.