From Leptonic Systems:
AirMote for iPhone lets you watch video, listen to music, and control presentations on your Macintosh, using your iPhone or iPod Touch as the remote control.The basic idea is you can sit back and control Front Row or other media players to watch movies, show photos, listen to podcasts and music around the house, or stand up and control Keynote or PowerPoint and give a presentation, or demonstrate your software to your audience from across a conference room or auditorium. Several remote control layouts are built-in, set up for controlling these applications. Other layouts take advantage of Apple's Mouse Keys, Full Keyboard Access, and VoiceOver features. AirMote is customizable, so it can command your computer in all sorts of ways. It does so by sending keystrokes to your computer. You can make your own buttons that send what you want, and put the buttons onto remote control layouts you set up.
Airmote requires iPhone iOS 3 or later, and so works well on older iDevices such as the original iPhone. If you have a recent iPhone/iPod Touch, or an iPad, consider Airmote II, even more customizable with more features, such as the ability to see and interact with the remote computer's screen.