WinCDEmu is a free tool which allows you to mount ISO, CUE, NRG, MDS/MDF, CCD and IMG disc images directly from Windows Explorer.
In theory, all you have to do is double-click the file in Explorer - or right-click it, and choose "Select drive letter & mount" - and WinCDEmu should automatically allocate it a drive letter. Accept this, or choose a new letter if you prefer, click OK and the image will become available from Explorer.
In practice, though, life isn't always quite so straightforward. When we first installed WinCDEmu, for instance, double-clicking ISO images had no effect at all, and there was no "Select drive letter & mount" context menu option.
Fortunately re-running the installation program and adjusting its settings restored normal operations, and we successfully mounted a series of images.
And as a bonus, WinCDEmu can also create an ISO image from any optical disc. Just right-click the drive in Explorer, select "Create ISO image" and give it whatever file name you'd like to use.