UFRaw is a decent and opensource RAW file utility. It works on many platforms and despite the ’80 looking site works quite well with NEF file from Nikon cameras. Moreover, it is fairly well integrated with The GIMP, thanks to a dedicated plugin. Setting it up on Windows may be tricky, especially regarding the Gimp integration. I found out an interesting post from with a workaround. It worked for me fairly well.
Thanks zirneklitis for sharing this.