I was listening to the Linux Voice podcast, Season 2 Episode 05, yesterday. They talked about their finds of the fortnight and one was Easy2Boot. A software that allowed booting from multiple iso's from one flashdrive(Windows, GNU/Linux, and even BSD). I have since been looking for an Open Source alternative largely because it would be useful, and with the help of alternativeto.net I found a few. There is YUMI, multibootusb, & MutliSystem. I have heard of and used YUMI previously due to Universal USB Installer. I have been too lazy to actually try it out. Multisystem is its own iso and must be booted from in order to use (they do have .deb's available for download although they could possibly be hard to find). Multisystem, when using the iso is all in French so the language must be changedd in the actual software and the Ubuntu settings. Only multisystem has the option to use GRUB (GRUB2 in this case) while both YUMI and multibootusb use the syslinux bootloader. However, my favorite is definetly multibootusb because there are more features and you also have the ability to virtualize in Qemu without a reboot. This is something I plan to use allowing me to only carry around one flash drive at a time for all of my live GNU/Linux distros.
Links
YUMI: http://www.pendrivelinux.com/yumi-multiboot-usb-creator
multibootusb: http://sourceforge.net/projects/multibootusb
mutlisystem: http://sourceforge.net/projects/multisystem
multisystem .deb's: http://sourceforge.net/projects/multisystem/files/apt/dists/all/main