Almost universally, serious digital photographers have replaced the darkroom with Adobe Photoshop, Adobe Lightroom, or Apple's Aperture. For the most part, Photoshop still dominates the market.
Through programs like Photoshop you can adjust a great many details of the image. One of the simplest with the most dramatic effects are correcting the balance of the levels. Observe the following two images:
While Photoshop and other programs tend to have an auto levels option, I have found manually correcting the levels to be more effective. Just open the levels window and pull the outer triangles to the edges of the histogram. This essentially resets the white and black colors of your image and then gives you a better distribution of colors.
Software can also be used to correct color balance, noise, remove imperfections in the picture and sharpen a soft image.