Windows: Chameleon is a free Windows utility that changes window colours based on the weather, the time of day, or battery level. When you're working on your laptop, this give you an obvious visual cue when you need to plug in without having to constantly check the system tray icon.
This free, portable app allows you to focus on using your computer, safe in the knowledge that a very obvious screen change will alert you to battery level. You can configure Chameleon to change desktop colours based on various factors including time, the weather, or program icons, but the battery option is particularly useful.
Click the Battery button to the left and you can then choose the colour that should be used to tint windows at different power levels. Default colours are pre-configured starting off with a calming green when battery levels are high, fading up to orange and red warning tones when you're running out of juice. Click any of the percentage boxes to make a new selection if you want to use different colours. Click Close to activate the settings and you can stop glancing down at the battery meter every few moments.
If you want to try out weather-activated colour changing, click the Weather button instead, and click the gear icon to the upper right to set your location. There's also an option to change window colour throughout the day based on time.
Chameleon runs on Windows Vista and above, and requires.Net Framework 4.0.