Android: If you aren't satisfied with how Android controls your device's screen brightness (is anyone?), Lux is a simple alternative that is easy to configure, while still offering a lot of options.
The concept is a lot like previously-mentioned Velis Auto Brightness, but it's a lot less complicated to figure out. The app launches as a small popover that lets you drag a slider to adjust your screen brightness. Once you're satisfied, you can lock it in as your default setting for that specific amount of ambient light.
From then on, Lux will work in the background to adjust the screen to your specifications as lighting conditions change. The more data points you give it manually, the smarter it becomes.
You get all of this in the free version of Lux, but the full app adds a lot of extra features. First and foremost, it will let you set brightness levels below Android's default minimum brightness, which is great for reading in a dark room before you go to bed. You can also tell it to change your screen's temperature at certain points of the day to help you avoid eye strain. To put it simply, Lux kicks the pants off of Android's stock brightness manager, and it's worth every cent.