Aperture, Lightroom and iPhoto each have dedicated sliders for enhancing the level of detail in an image. None of them can recover fine detail that wasn’t captured initially, but they can increase the apparent texture, crisping up contrasts and edges in a more subtle manner than the dedicated contrast tool.
Use them with care, because it’s easy to over-bake the effect and introduce haloes around sharp contrasts. These are easy to spot, and it doesn’t take a trained eye to realise that they point to someone using amateur editing to compensate for substandard photography.
Pixelmator doesn’t have a dedicated Detail tool, but you can use the Sharpen tool (from the tools palette) to paint over areas that you want to make sharper. Applying this to eyes in a portrait shot can make a big difference, and immediately draws the viewer’s attention.
If you’re working with raw images from a regular camera or DSLR, you can enhance detail within Adobe Camera Raw when importing shots into Photoshop, but if not then try duplicating the image on a new layer, applying an high pass filter on the duplicate (Filter > Other > High Pass) and then changing the blend mode of that layer to Hard Light. You can then use the opacity control or layer masks on the new layer to adjust the strength of the detail enhancement within the overall frame.