Nikon Capture NX is an altogether more ambitious and effective program than the Canon's free software, Digital Photo Pro.
It's designed as a general-purpose image-editor, not just a RAW converter. It only converts Nikon NEF files, but it does it very well.
Only works with Nikon files
It can mimic the camera's own colour and tone settings, offers highlight recovery, lens distortion correction, a full range of noise-reduction and sharpening tools.
You can apply Nikon's D-Lighting tools to control shadow and highlight brightness, and the unique 'U-point' system makes it easy to tweak small areas. It's an interesting alternative to Photoshop.
Nikon SLRs seem to produce very good JPEGs, and there's little difference in sharpness between an image processed in-camera and a RAW file.
The highlight recovery is a big bonus, though, and the Auto Color Aberration checkbox removes almost all traces of colour fringing.
Plenty of image adjustment options
You can crop and rotate photos as well as correct lens distortion, and the point is that all these changes are reversible - they're applied via a set of 'stacked' adjustments, each of which can be revisited.
And if you save the edited image in the NEF format, your adjustments are saved too and visible when you re-open the file. You can, of course, save TIFFs or JPEGs too, though you lose the ability to change your adjustments later.
Capture NX is a very powerful application that not only carries out Nikon RAW conversions extremely well, but makes a very good image-editing application in general.