NEF (Nikon Electronic Format) is the format that Nikon digital cameras use to store images in raw form (as it is, without any compression or transformations applied). Not all graphics software can view this image format. If you own a Nikon camera it would have come with required conversion software to convert NEF files to other popular image formats such as JPG and BMP. In the absence of such conversion software, you can use Picasa image viewer from Google. Using Picasa you can view NEF images as well as save them in JPG format. Picasa image viewer can be downloaded as part of Google Pack.
Do you know any other software that can handle NEF images? Please share them in this page.