The best day of Photoshop class was when I learnt how to fix a blue colour cast in a picture. That's when the blue sky makes everything in your picture look a bit blue. The same goes for tungsten bulbs making things look yellow, fluorescent lights and sick green, and cloudy weather and washed out grey tones.
So all you have to do is fiddle around with levels. This is very much a trial and error process. Go to Image>Adjustments>Levels... or just hit Ctrl L. If you want to make the adjustment in a separate layer, go to Layer>New Adjustment Layer>Levels...
In the Levels Panel you will see a funky graph. The far right slider controls light tones, the middle one mid-tones and the far left one dark tones. If you have a flat area on the left or right of your graph, try moving the slider in towards where the graph starts. This is a great way to improve especially under-exposed pictures.
To remove a blue or other colour cast, look at the dropper buttons on the right side of the panel. Choose the middle one and then click around, aiming for the best neutral grey pixel in your picture. After each click just hit Ctrl Z if it doesn't look good. If you can find a good neutral grey thing, your colours should correct nicely. Shadows on white objects are often good.
My problem lately has been hazy landscapes. No matter how I expose them, they just come out pretty darn hazy. So in the picture below I used the black dropper and went hunting for something that should be black; as opposed to hazy grey. Again I found that neutral colours work best, e.g. the dark part of a building, not the dark part of the forest. In this particular example I had to adjust the far right slider as well, and well, here's the result!
You might notice that the trees in front look a bit upset by the process, seeing as how they were the only thing that looked good in the original. But all for the good of the landscape!