I have tried Capture NX-D, Capture NX-2 and the DxO Optics Pro 7 (the newest version is 9, but 7 can be downloaded for free till the end of this month).
Of these applications Capture NX-2 has a very nice tool called Control Points which provides an easy way of manipulating selected features of the photo. E.g. you put a control point on someone's face and make it a bit brighter, select the sky and make the blue more saturated etc. I find this an easy way to adjust the photo a bit (I'm an amateur myself, I bet pros would find this tool a bit limited).
As AJ Hendeson already mentioned the Nikon software provides options well suited to Nikon cameras but the main functionality of other tools is similar.
In my personal opinion the Nikon applications have rather poor user interface (NX-D being the worst and slowest), you might want to take this into account. On the other hand this is highly subjective so your opinion may be different than mine.
I like DxO's user interface the most but overall I find NX-2 with its Control Points most useful.
Nikon Capture NX-2 has a free trial so you may test this application before you decide to buy (same goes for other software like e.g. Adobe Lightroom).