Mobile Mouse Pro (iPhone/iPad, $1.99)
Mobile Mouse Pro works for Mac and PC, becoming a remote trackpad and mouse, a remote control for your apps, and a presentation controller. Works over wifi and requires a server to be running on your computer.
How to use it: Grab the software from their site, install and run the server, configure settings, and use the app.
Gmote "turns your Android phone into a remote control for the computer". It lets you control music and movies with standard remote control features, has a built-in file browser, as well as touchpad and keyboard functions.
How to use it: Grab the server software, set up a password, and set up your media paths. Simple as that. Gmote can run over both 3G and WiFI.
TouchPad (iPhone/iPad, $4.99)
Touchpad becomes a remote control, extended keyboard, multitouch trackpad and remote. It runs through the Mac OS X built-in Screen Sharing, so requires no additional software for the computer.
How to use it: Enable Screen Sharing in your System Preferences, run the app, and follow the prompts. Online support is available at their site.