Levels and Curves: Photoshop's Left and Right Ventricles Pump Luminance

Photoshop Top 40, Feature #9: Levels

The Levels command, and its cohort the histogram, let you adjust luminance levels on a channel-by-channel basis. The upshot is that you can increase contrast, correct for color cast, and make a bad image good.

Photoshop Top 40, Feature #16: Adjustment Layers

An adjustment layer is an independent layer of color adjustment that you can edit any time you like. Plus it affects all layers below it, consumes very little space in memory, and affords the opportunity for selective edits.

Photoshop Top 40, Feature #28: Hue/Saturation

Hue/Saturation not only spins colors and intensity values, it lets you modify one color independently of all others. And it does so in such a credible way, you would never know it was there. Perfection.