ISO image mounter/Virtual optical drive (Daemon Tools)
Given the amount of software testing I do, I download software all the time. A lot of what I need comes in the form of ISO files. So one of the first tools I install on my new Windows systems is an ISO image mounter called Daemon Tools Lite. Although Daemon Tools can do a bunch of other stuff, I use it just to mount ISO images on virtual CD/DVD drives that the software creates. This allows me to load an ISO as if I were inserting a disc into a CD drive.