WinCDEmu is an Open Source CD/DVD/BD emulator that allows you to mount optical disc images by just clicking on them in Windows Explorer.
In other words, if you have an ISO image and want to use it without burning it WinCDEmu can do it.
While the setup may vary slightly from different versions of Windows, for the most part, you need to allow WinCDEmu to install a driver for emulation. Then you can mount ISO, CUE, NRG, MDS/MDF, CCD and IMG directly from Windows Explorer as seen in the screenshots below. You can also build disk images from the same menu.
WinCDEmu features one-click mounting of ISO, CUE, NRG, MDS/MDF, CCD, IMG images, unlimited virtual drives, the creation of ISO images through a context menu, no rebooting, a portable version, 20 languages, and it supports data, DVD-video, and BD-video images.