Windows: Dual (or triple or quadruple) booters know how annoying it is to wait around for the boot manager when rebooting. Free system tray utility iReboot lets you pick your OS as you restart, so you can go make a sandwich instead.
Screenshot from The How-To Geek.
Whether you've got multiple versions of Windows on your computer or even a Linux partition, iReboot will help you skip the boring phase of sitting at your computer and reboot straight into your chosen OS for you. All you need to do is right click on the system tray icon, pick your operating system, and iReboot will do the rest — so you don't have to sit around tapping your foot.
I wasn't able to test this out myself, since each of my partitions have their own bootloader, so let us know how it works for you. Also, while it was designed to work with Windows' boot manager, it makes no mention of GRUB if you use that as your main bootloader, so give it a whirl and let us know what set-ups do and don't work for you in the comments!