Check out Anastasiy Safari’s new MixColors panel for Photoshop:
It provides an engine for fast and correct color mixing, like oils, acrylics or watercolor. It keeps color history, allowing you to keep a number of swatches (like it’s on real-world palette). In “Mix eyedropper mode” the colors you pick with Photoshop eyedropper are mixed to the current color with one click. It also can be “attached” to my MagicPicker so the two panels act as one, plus more features.(As usual, CS3, CS4, CS5 and CS5.5 are supported).
The panel is on sale for $14.