The breadth of software options for improving photo quality is impressive. Many solutions are plugins for Adobe Photoshop or provide a user friendly interface for what could be achieved with complex settings among default Photoshop tools. A more affordable contender to Photoshop is Pixelmator, previously discussed on this site. It has not been around long enough to establish itself and has some professional limitations but offers an impressive interface nonetheless.
Topping the list of enhancements is a noise reduction alternative to the Photoshop implementation. Neat Image is a worthy plugin that works on RGB or grayscale photos. It analyzes noise patterns of camera models, adjusting for ISO settings by reading a sample area. Sliders are then used to diffuse the noise pattern. Imagine a checkerboard. This plugin figuratively recognizes the pattern of red squares, expands the selection, and softens the edges in such a way that, from a distance, the board resembles, depending on settings, deep red or even pure black rather than individual two-color squares.
Arguably, what may be the Cadillac of noise reduction is Dfine from Nik Software. Its U Point technology makes noise reduction intuitive. Results are achieved without relying on camera profiles. Once you get accustomed to U Point, you'll no doubt consider it for the next important area of photo editing, color refinement. Nik Software implements this well in Viveza. Rounding out the final area of photo enhancement is a sharpening tool. Again, Nik Software responds with Nik Sharpener Pro that comes in Inkjet and Complete Editions. Many alternative plugins and Photoshop Actions are available on the Web. Lightroom and Aperture are competing Photoshop companions from Adobe and Apple respectively.
For those focusing their efforts entirely on faces, Portrait Professional attempts to combine all necessary tools within a somewhat awkward interface. Though impressive results are shown its gallery, three-quarter view, closed eyes and mouths, or tightly cropped portraits may not fit the parameters for automatic enhancement. Color control is less intuitive, producing sub-optimal results for challenging images. Despite the name, professionals will likely use one of the other offerings.
See: Big Pixel Workflow