This one goes out to all the night owls. As much as I love my iMac, I was surprised to see how little control it gave me over the screen's brightness. I often work on my iMac late at night in an otherwise darkened room, and even at the lowest brightness setting, the iMac is really, really bright. All of the Mac laptops I've used have the ability to dim the screen down to completely dark, but the iMac does not.
That's where Shades comes in. Shades is a free preference panel which gives you the ability to control the brightness of your Mac through the menu bar or a floating slider control, or even keyboard shortcuts/hot keys: for example, to increase brightness when Shades is running, I can simply use the option/alt key and the up arrow (or the down-arrow to dim). I can also enable or disable Shades entirely or open the menu bar controls, all via the keyboard.
Shades will let you set a minimum brightness level so you don't accidentally make the screen so dim you can't see anything (although if you have enabled the Hot Keys feature you can use the keyboard to increase the brightness even if you can't see the screen). I recommend setting a minimum brightness that's still visible, especially if you have set your Mac to require a password on waking from sleep.
Shades supports multiple monitors, giving you independent control over each one. While I think this is a good feature (each monitor can have its own setting) the keyboard commands only change the main monitor. You have to adjust the secondary monitor's brightness with a slider control, which is easily done through the menu bar. However, if you have two monitors and have both of their controls in the menu bar, there is no easy way to know which one is which. There is a workaround for this: go into the Shades preference panel and click the tab for one of the monitors and set the "Menu control" to "Shades Icon"; then go to the tab for the other monitor and set the "Menu control" to "Brightness Icon". Now you will have two distinct icons in the menu bar, making it easier to know which monitor's brightness you are adjusting.
You could also use the "Menu control" for one monitor and the "Slider control" (which floats on-screen) for the other. The ability to sync the brightness levels of both monitors would be nice, but this is a very minor complaint. My external monitor has no brightness controls whatsoever (and I know other monitors which make you hunt through a series of on-screen menus) so without Shades I would have no way of controlling its brightness level.
Most laptop users may not need Shades, but it does offer finer control than what the built-in controls offer. If you're an iMac user (or a Mac Pro user with a monitor with hard-to-access brightness controls) who works late at night, give Shades a look. When you're done, simply quit Shades and the monitor will return to its regular brightness level.
Shades runs on either PPC or Intel Macs, and requires Mac OS X 10.5 Leopard or later.