Mac: F.lux is a great app for changing the temperature of your display when you're working late at night, but working late at night also tends to mean you're skipping sleep. The Mac version of the app now has a new "Backwards alarm clock" that counts down the time until your alarm goes off.
The new feature essentially just tells you how much sleep you'll get if you go to bed at any given moment. It's a little thing, but considering most of us only use F.lux when we're working late nights, it's actually really helpful. Beyond that, the Mac version also gets a new option to dim your screen slightly when you disable F.lux, better transitions and more daytime settings.
F.lux for Mac (free)