The Color Wheel
Hue - color (red, blue, seafoam green, etc.);
Value - lightness or darkness of a hue or grayscale
Saturation - intensity of a hue dependent upon the amount of gray
Contrast - difference in value between two hues and/or grays
Basic Color Theory. Color Wheel, Color Harmony, Color Context.
Hail Pixel. Tool that allows you to view color characteristics including hue, value and saturation. You can save color schemes.
Color Hexa. Encyclopedia of color.
Color Scheme Designer. Enables you to sample various color schemes including monchromatic, analogic, triadic, etc.
Color Exercises. Also, an introduction to color theories and models.