YUMI is a portable free tool for creating multiboot USB drives. It's able to take a number of bootable images - Linux distros, antivirus, partition managers, recovery tools, forensics, more - and combine them onto a single drive. Boot from this and a menu allows you to choose the tool you need.
Setting this up starts by downloading your first boot image. Look for it in the Distribution list (options include Ubuntu, Linux Mint, Fedora, AVG/ AVIRA/ COMODO/ F-Secure/ Kaspersky rescue discs, Ultimate Boot CD, Partition Wizard and many more), follow the link to visit the home page, or check "Download Link" to download the ISO right away.
Once you have the image, point YUMI at your destination USB drive, click "Create", and it'll quickly be configured.
Now just repeat the process to select further images. YUMI adds them to your drive, rather than replacing what was there before, and you'll soon have your own custom rescue disc.
Better still, this doesn't mess up your drive with odd partition layouts or anything similar. For the most part, everything is tucked away in just one or two folders, and, if you've any space left, you can still use it to store data as required.
Version 22.214.171.124 (changelog):
Quick update to add support for newer System Rescue CD 6.0.2+
YUMI provides a quick and easy way to build multiboot USBs