For people short on SD cards: Berryboot is a simple boot selection screen, allowing you to put multiple Linux distribution on a single SD card.
In addition it allows you to put the operating system files on an external USB hard drive instead of on the SD card itself.
Download link Berryboot for the Raspberry Pi: berryboot-20121230.zip
To install: extract the contents of the .zip file to a normal FAT formatted SD card, and put it in your Raspberry Pi. This can be simply done under Windows without any special image writer software.
Once you start your Pi it will start an installer that reformats the SD card and downloads the operating systems files from the Internet.
Link | Berryboot