Mac only: Inspired by our previous post on remote Mac muting, Brett Terpstra created a remote server package and iOS-friendly web app that allows for remotely controlling your Mac's volume, iTunes functions and even launch applications from anywhere.
Installing Brett's HomeControl package requires a bit of Terminal work, as well as some understanding of how your Mac and iPhone connect to the network. Beyond that, though, it's a nice, condensed web package for remotely controlling your Mac from a browser. Those willing to hack around in the HTML can add iTunes functions, applications to launch and other features, but the basic suite is pretty handy in itself.
HomeControl is tailored for remote control through iOS-style browsers on the iPhone and iPad, but you can likely get by with any browser on your phone or wireless device.
HomeControl: Local Mac control for iPhone [Brett Terpstra]