Specifically not interested in renting software, so Photoshop is a non-starter. I'd pay for something, but I'm finding the free alternatives to be capable.
First-off, I'm going to work with the RAW (Nikon: NEF) files to produce 16-bit TIFF files for further creative work. If you're just going to work with the JPEGs right out of the camera, your alternatives open way up. For that, I just use Picasa; it's really simple and gives good results. I have also used GIMP for more involved edits; it's more complicated, but it does just about everthing Photoshop does, and has a robust community developing both the core product and filters to do a lot of things. They even have scripts emulating some of the more arcane Phtoshop tools. There is a RAW plugin to allow opening such files directly in GIMP, but it's not easy to install.
If you're going to start with JPEG, just know that you're starting with an 8-bit colorspace, so if you do too much modification, it starts to show.
My primary choice for RAW conversion so far is darktable. It takes your RAW file and produces a separate file capturing your modifications, so no edit is 'destructive'. It's got what I consider to be a suitable set of tools for making the data out of the camera presentable. Disadvantage is that it's Linux (and Mac, it think), not Windows. Specifically not Windows, the devs aren't interested in that OS.
I'm also going to look at ViewNX, the software that comes with the camera. Seems to do most of what darktable does, I still need to investigate how it stores modifications. It runs on Windows and Mac.