Keep Drive Spinning ensures that a connected drive never goes into low-power mode. Normally this can be controlled using the Energy Saver preference pane in System Preferences, and that should be your first resort in dealing with the problem, rather than this or any other third party software. However, some external drives do not respect the System Preferences settings, so for them an alternative like this is needed.
This applet simply creates a launch agent that tells OS X to update a hidden file on the selected drive once every minute so that the drive stays awake. Launch agents are built-in OS X technology and persist across reboots, etc., so you can run this once to set-it-and-forget-it. You can find the launch agents made by Keep Drive Spinning in the LaunchAgents folder of your user’s Library folder.
Users on OS X 10.4 (Tiger) and OS X 10.5 (Leopard) need to run the AppleScript source code directly. This source code is included, and only requires opening the file and then clicking the big, friendly Run button.