I added this gadget to my toolkit a couple of months ago and find it extremely useful: Zalman ZM-VE200
. At first glance it looks like an ordinary external HDD case. However, it has the ability to emulate an optical disk drive.
When you plug it in to a USB port you can select from 3 different modes: HDD, ODD or Dual. If you select ODD or Dual you will then be able to scroll thru a list of ISOs that you have copied onto the disk (such as a Win 7 system repair or install ISO). Once you select which ISO is "loaded" you can then access it as if it was a CD\DVD in a drive. Of course you could do that with software such as Daemon Tools BUT, unlike Daemon Tools, you can set the system BIOS to boot from USB and boot from the selected ISO.
Now, instead of carrying a stack of CDs or DVDs with me to do installs, repairs or whatever, I carry this little drive. I put a 160G drive in the case and currently have 30 ISOs on it that I can choose to boot from with 130G still free on the drive.
Using the Dual mode, it will appear as an optical drive AND an HDD which allows you to access other software such as 3rd party installers and tools that don't need to be accessed via an ISO.
This is almost as convenient as a thumb drive to boot from but instead of being able to boot a single OS you can choose from many. It's much easier to setup too. No special prep needs to be followed to make it bootable like if you were preparing a thumb drive, just copy any ISO to the folder on the drive named _iso and it's now available. I have Windows install ISOs, different Linux distros, repair disks, boot disks, etc... on mine.
My only complaint, so far, is the selector switch. It's used scroll thru the list of ISOs to select which to load and can be a little quirky to use. It's a thumb wheel kind of a switch where you toggle up/down to scroll up/down thru the list and press in to select. However, it's very finicky about being pressed and when I press to select with my fat fingers it usually takes me a couple of tries for it to select rather than scroll up or down.