I am aware of several 3D paint tools of WIDELY varying quality and price. Very good quality but costly tools for 3D paint include DeepPaint and BodyPaint. Maya has basic 3D paint functionality built in via its Artisan toolset. The only free 3D paint tool I know of is Chameleon (and it is barely a programmer's experiment in 3D paint:)
http://www-ui.is.s.u-tokyo.ac.jp/~takeo/chameleon/chameleon.htmThe next best thing to real 3D paint tools are tools like Ultimate Unwrap which update the texture on the model automatically as you work on the texture image. Ultimate Unwrap allows you to setup a floating window with a textured view of your model, set that window to Always On Top so that it is visible while working in a paint app like Photoshop, and then the texture is automatically updated whenever you save the image. Thus you can see the texture applied to your model while you work on the image.
Other tools of that latter ilk include Tru-V and Amazon Paint.
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I've never heard of NST before; from searching on Google I am guessing you mean NPherno's Skin Tool.