Match Color

With Photoshop CS2, Adobe added the Match Color feature which lets you alter the palette of one image to look like another. Match Color can be used for everything from slight tonal corrections, to ensuring that an image fits better with a design scheme or other imge. unfocusedbrain.com, a "blog with everything" has an excellent demo of how you can use Match Color in conjunction with famous classical paintings to perform dramatic color adjustments. Check it out here.