Try Nikon's ViewNX utility. This is a free utility and should be able to open and convert RAW files from any Nikon camera to either JPEG or TIFF.
Another alternative free tool I use is UFRaw. This also works with pretty much every DSLR RAW file out there, but you need to get the camera profile's ICM file for it to properly render the colour and levels. The website lists a few ICM files for older cameras, but hasn't been updated in a while, e.g. no profile listings for recent models such as the D3100, D5100, etc.