Windows only: Generally, to save power, we sleep or hibernate our computers when not in use. But if you want to keep your computer awake at specific times or when certain programs are running, free program SmartPower will schedule it for you.
There are a number of ways to keep your computer awake with SmartPower. The easiest is a simple time schedule: for example, the screenshot to the left shows a schedule of the computer waking up at 10pm and staying awake until midnight. You can also tell it to stay on when certain network devices are connected, when your network throughput is above a threshold (say, if you're seeding or downloading torrents), when your CPU is above a threshold (say, if you're encoding video), or when certain processes are running (for pretty much anything else). Thus, no longer do you have to manually tell your computer to avoid hibernation every time you have a lot of tasks that take a long time to complete — you can just set it to stay awake when it's working, and then it will go back into sleep or hibernation when it's done.
SmartPower is a free download, Windows only.