Everyone knows Cron for running reoccurring tasks at specific times. But what if you just want something to happen once, like switching a site to live, or restarting a computer due to the latest sets of patches? You could make a complicated cron that runs and then removes itself or you could do it the easy way. Enter At, I feel the unknown sibling to Cron. At runs a command once, you give it the command (or series of commands) and when you want it to happen and it does the rest.
See examples here: