Backlight! is a free brightness app/widget for Android that changes your devices’ screen backlight with a single click. It also allows complete custom brightness levels. You can even assign it to a gesture or a main dock for easier access.

NOTE:
1. Doesnt work on Sense UI devices (i.e. Hero).
2. Potential conflict with Juice Defender. JD seems to negate any requests of this app to change brightness.

Android recognizes values from 1-255 with 1 being darkest and 255 being brightest. It also has support for automatic brightness. Default levels are 1, 50, 150, and 255 respectively. To enable Auto Brightness you can use the letter 'a'.

Backlight! controls the screen brightness level only. It does not adjust keyboard brightness or other light features of the phone.

Upgrading?
Don’t forget to add/re-add the widgets to the home screen!