Anyone who runs more than one operating system has had to deal with GNU GRUB (the GRand Unified Bootloader). Grub is the tool that allows you to pick which operating system to book when you turn your computer on. But you can do a ton more than that by configuring it and derailing from the standard configuration. Unfortunately, until recently users were forced to open up cryptic config files in text editors and try and figure out what to do based on the comments (or, more diligently, by reading man pages).