I seem to recall that I can decompress RAR files with the provided stuffit stuff. Do we not have a compression mechanism without a command line utility? Doing a front-end for that would be easy enough.
A simple combo of DynDns and Chicken of the VNC for controlling someone else's computer over the internet, its easy with bonjour on the same network, not so easy at another place.
So, give me some cool ideas and see if you can inspire me to make a new Cocoa app. Hey, maybe some of the other OSX developers on the forum could find inspiration if we get a good thread going.