Scott Kelby - Using Curves for Color Correction
Life Hacker.com - Basics of Photoshop: Color Correction, Touch Ups, and Enhancements
Photoshop Essentials.com - Fix Tone and Color with Levels
TutsPlus.com - Adjusting Color with Curves
The Design Cubicle - This tutorial shows you an advanced method of evaluating your photograph’s color accuracy and making the proper color correction adjustments. It is a more sophisticated technique of removing unwanted color casts, but the result’s will knock your socks off.
Photoshop Cafe - This technique is a contrast/tone and color correction technique all in one. Once you run through this, you will see how just about any image can be improved and you will use this technique many times over.
Photo.net - Using Lab Color to make color adjustments