Files with an .NRG file extension are used by the CD/DVD burning application "Nero Burning Rom", and are exact images of the original CD/DVD.
If you do not own Nero you can open the file using a CD/DVD emulation tool, such as Daemon Tools or Alcohol 120%. These tools allow you to create virtual CD/DVD drives on your computer, allowing you to mount the .NRG image and use the CD/DVD as if it were in your computer's CD/DVD drive.
For Linux uses, there is a free tool to convert .NRG files to the standard .ISO format (see links below).
If you know of any free Windows CD/DVD burning software that can read .NRG files, please let us know by using the comment form below.