WinSetupFromUSB is a free tool which can create bootable USB keys from various ISO images.
As you'll guess from its name, the program started life as a way to prepare USB keys to install Windows. It still does that very well, too, for every version from Windows 2000 to 8.1.
WinSetupFromUSB now also works with many Linux-based ISOs, though - Debian, Ubuntu, Fedora, CentOS and more - which also gets you support for many antivirus rescue disks. The program can also work with PE and PE2-based images, Syslinux/ Isolinux-compatible discs, and anything else which can handle grub4dos CD emulation (Ultimate Boot CD, Acronis and Paragon boot disks and so on).
Best of all, WinSetupFromUSB is able to combine any of these images into a single disc. Simply point the program at the ISOs you need and it'll create a USB key with a boot menu where you choose whatever you need.
If you're thinking this sounds like a lot for one program to handle, then you're right. Fortunately WinSetupFromUSB's developers know that, too, and they use a host of other free and open source programs to do much of the hard work (RMPrepUSB, BOOTICE, ImDisk, QEMU, FbinstTool and more).
We noticed one or two oddities here. Our first attempts to create a bootable key failed with some error about the MBR, for instance, and instead of allowing us to tweak our settings and try again, the program just closed.
Still, once we'd got around these problems WinSetupFromUSB worked just fine, and if you need a versatile way to combine bootable ISOs into a single USB key then it's certainly worth a try.