Photoshop is always the mainstay, but it's far more expensive than Aperture, which is actually on the cheap side as far as good programs go.

Aperture, though (and Adobe Lightroom) are more RAW converters and "Digital Asset Management" software than full scale editors.

Most people who stop short of Photoshop do go for Photoshop Elements or Paint Shop Pro.

Nikon Capture NX is another very good program, and licenses will be pretty easy to find as Nikon is giving them away free with Nikon D3 and D300 cameras- and many of those buyers are either getting both or already have NX.