Although other Photoshop users may disagree, I think the two most powerful tools in Photoshop are "Curves and Color Balance.*"

The first adjustments I make to any image is to my curves and my color balance settings. Curves provide a visual graph that allows you to adjust not only the highlight, midtone, and shadow values, but also allows you to change any value at any point on the graph. Blah blah blah, what does this mean? In simple terms, this tool is a rich man's Brightness/Contrast. You can use a Brightness/Contrast tool, but it is very limiting and gives you very little control over the image. Don't take this shortcut, learn how to use Curves.

The settings in Color Balance allow you to work with the highlight, midtone, and shadow color values. Again in simple terms, this tool helps you get the color that you want to pull out of an image. Now I am not going to spend hours boring you with how to use these tools and a step-by-step of how I got the images here to look the way they do. There are a lot of excellent image editing books and web resources. I can tell you that EVERY image here (with the exception of the two final images) was improved using only these two tools. I didn't spend hours editing these images and making them perfect. Instead, I focused on 2-3 minute corrections to show how a few moments of your time can make all the difference.

*Some image editing applications have color balance settings built into their Brightness/Contrast tool. If that is the case with your application, you actually would use that tool.