It is hard to pick just one, but I guess I will go with QuickLaunch because it not only is in the too three or four of my "can't live withouts," but it is also the most freqyently used. Basically, it is like an extension and enhancement of the OS, which really makes it most used and would probably make it the most missed if it weren't there. Kind of like removing the wheels from your car.
Posted from my CrackBerry at wapforums.crackberry.com