Apple's Magic TrackPad is available, but many people do not want to spend $70 on it.
This instructable will show you how to turn your iPhone or iPod Touch into a trackpad for your computer.
Step 1: Requirements
A PC running Windows XP, Vista, or 7 or a Mac running OS X Snow Leopard. XP will be used in this tutorial, but setup is not that different on other operating systems.
An iPod Touch / iPhone
A WiFi network that your PC / Mac and iPhone / iPod Touch are connected to.