NEF files are Nikon Raw Picture files (there's no compression) - click here for XnView which can open and convert .nef files. You can then convert them to another good photo format - TIF. These can then be opened with PhotoShop and edited. You can also convert them to JPEG, but this is a lossy compression and can decrease the image quality. You can also click here for a site that has information about other file extensions and what programs can open them.