Midnight is a widget for the Mac's dashboard widget engine that gives you the ability to put your Mac to sleep with the click of a button. To be honest, if that's all this widget did, it wouldn't be worthy of a post, but this one does one more interesting thing. It can hibernate your system, much like a Windows computer can hibernate.
Most Mac users aren't even aware that their Macs are capable of hibernating, although they may have heard the term "deep sleep" before. A sleeping Mac is analogous to a Windows machine that is in Standby mode. While it is technically off, a small amount of power is being drawn to keep the system's memory alive. In Windows, you have the option of choosing to go into hibernation mode, which takes a bit longer since all of the contents of your system's memory are then written to the hard drive. This allows the system to turn completely off and draw no power, since unlike RAM, the hard drive does not require power to retain information.
A Mac's deep sleep mode is like a PC's hibernation mode. The big difference is that Macs don't expose this to the user, but rather as might be expected based on Apple's "simplify everything" philosophy it simply decides on its own when to use each mode.
So if you're a control freak and would like to save a bit of battery life when stuffing your MacBook into your backpack and heading out on the road, have a look at this little dashboard widget that lets you force your machine into deep sleep mode. You'll have to turn on the configuration option the first time you use it, but after that you can sleep a little easier that your Mac's battery isn't wasting precious energy keeping your Mac in sleep mode when it ought to be in a deep slumber.