ImgDrive is a free tool which can mount multiple disc images as virtual CD, DVD, HD-DVD or BD drives.
The program supports from 1 to 7 drives. You're able to set the number of drives you need from ImgDrive's system tray icon, or disable the feature on demand.
ImgDrive's system tray menu lists each of these drive letters in turn, and you can select one, mount a new image (iso, ccd, cue, isz, mds and nrg formats are supported), or dismount the current image as required.
These images can be automatically mounted, if required, meaning each virtual drive is restored when Windows reboots.
There's also an "Integration" option, which in theory enables images to be mounted from their Explorer right-click menu. This didn't work for us, but we've no idea why: maybe you'll be luckier.