There’s no shortage of ways to root Android devices, but z4root is probably one of the simplest solutions I’ve seen since Universal Androot (which no longer works on some devices if you’ve upgraded to Android 2.2.1).
The app was built by xda-developers forum member RyanZA and all you have to do is load the app onto your phone, run it, and click the button. It will give you root access to your phone, allowing to you install and run apps that require root access. If you run into any problems, just reboot your device and everything should be back to normal, since z4root doesn’t make any major changes to your system files.
The bad news is that not every Android device is currently supported. While z4root works on a number of popular devices including the Samsung Galaxy S, Motorola Droid 2 and Droid X, and Sony X10, it doesn’t curently support the HTC Evo, G2, Magic, Desire, or Desire HD.
You can find a download link and more information about supported and unsupported devices at the xda-developers forum.