Wake Up is a very useful app, and its name describes its purpose. It's designed to wake up your computer after a specified period of time, keeping it from sleeping and blocking progress on a download or other task you've set. The app is very straightforward, developed as a personal tool by the programmer, but it is ultimately very useful for anyone that needs to keep their machine on at all times.
Installation is straightforward with the normal security issues that come with any third-party app on Mountain Lion. After overriding those settings and opening the app, you can set the time you want the app to wait before "poking" the machine and then turning it on. After turning the app on, it will count up to the timer limit, and then refresh the app when it reaches the time, before resetting to zero and starting over. The effect is that your computer will be "woken up" ever X minutes by the app. There are no additional settings to speak of, but the app works well and is a very useful tool for anyone that needs their machine to stay on, even when they are not sitting in front of it.
The point of Wake Up is to keep your Mac running at all times without having to change the power settings. It's especially useful when you have big tasks that you want to complete uninterrupted without your intervention. Even better, it's a free app that is easy enough for anyone to use.
From Custom Solutions of Maryland:
Wake Up pokes the display every so many seconds to keep the Mac from going to sleep. The User decides how many seconds between pokes using a drop-down menu (Let Me Sleep plus one to ten minute settings). The seconds are counted down and displayed in the Control Panel's top right corner. When the countdown reaches zero, the cursor is moved to the Control Panel and the mouse is tapped (simulating the User clicking on the Control Panel). The Control Panel includes a checkbox to pause the countdown.