Lightswitch is a software control for switching the backlight off and on. This is faster and easier than holding down the power button.
For PDA's not fitted with a backlight, it lets you adjust the brightness.

Version 2: Lots more functionality including:
Explicit turn backlight on ond off buttons
Show backlight status, whether off or on. Explicit Done button to exit
Works seamlessly with Torchlight to make it easier to use.

And its easier to install.

If you like this application checkout Light Power Pak. Apart from including lightswitch and Torchlight includes two new applications, LightOn and LightOff, that turns the backlight on and off respectively by a single tap to to application icon.

Software doesn't get any easier to use than that!