Phone Home is a Ruby on Rails web server that runs on your computer to remotely control specific applications from your iPhone (or other device). Phone Home currently supports remote controls for iTunes, DVD Player and Keynote. This application is more than a directional keypad; each remote is designed to operate with the respective application to give appropriate status feedback and self-updating screens. It is in early beta development so expect many more controls coming soon.
IGG SOFTWARE Be sure to checkout iPhone Remote as well. It is free and has some great apps working for it, http://code.google.com/p/telekinesis/ (Version 0.2)
LOUDESTNOISE This is so great. I will willing pay money for this. I loved Salling Clicker with my Nokia phone, and this a great replacement. Add some apps and I'll gladly drop $25 for this in a heartbeat. (Version 0.2)