If you need to install software from a CD or DVD onto a computer with no optical drive, such as a netbook, create a disc image from the installation disc, install DAEMON tools in the destination ...

Similar to previously-mentioned Daemon Tools, this is a great shortcut for anyone with an ISO on their hands without a DVD burner.

If all you need is a way to run .iso files, it's another freeware alternative that's not too complex. Far as I can tell, it'll mount up to 8 images at once.