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.
Step 2: Download HippoVnc
Run WinVNC.exe. Make yourself a password and click OK. You do not need to worry about any of the other options.
A new icon should appear at the bottom-right corner of your screen.
Step 3: Step 1: Download the HippoRemote App
Launch the App. From the menu select "Add Connection". Type in the IP Address of your computer, (which can be found by running your mouse over the HippoVNC icon in the task bar) type in you password and select 'Done'.