Mac only: If you have more than one startup volume on your Mac (say, if you're using Boot Camp), free utility QuickBoot lets you reboot from one of those volumes straight from the menu bar, with just a few clicks.
Ordinarily it's a bit of a pain to start up from another volume — you either have to go through System Preferences or hold Option when you restart your computer (and no-one likes to sit around and stare at a rebooting computer). QuickBoot sits in your menu bar (although you can also run it as a standard application to save menu bar space) and gives a list of all your bootable volumes and the OS installed on each — including, in the case of OS X volumes, version numbers. If you run it as a standalone application, you can even choose to reboot to that volume immediately or next time you restart.
The app isn't perfect; it doesn't work on some computers (such as the MacBook Air), and at the moment, it only recognises Mac and Windows partitions — it didn't recognise my Linux partition (although when I chose to boot from my Windows partition, it booted into my Linux partition — which is fine, because GRUB recognises Windows; it was just weird). Regardless of these drawbacks, if it works on your machine, this app should make life a bit easier.
QuickBoot is a free download, Mac OS X only.