Call Blocker

Filters both incoming and outgoing calls
Also blocks patterns (for example, all numbers starting with 800 or containing a certain number)
Very useful when lending your phone for someone else to use or block that person you don't want to talk to right now;
I personally use it so my daughter cannot call the people in my Contacts list when she gets my phone to play with.
Click on the "How To" button for an overview on how to setup the filters.