I vaguely recall using a color wheel in 9th grade art class. To tell you the truth, all I remember about it is that red and green are complementary colors (hence, Christmas). I do realize, however, that many people make frequent use of this reference. If you're among them, check out Old Jewel Software's Painter's Picker.
It adds an interactive color selector to almost any application via the built-in color picker. Now you've got a color wheel ready to go, which you can use to find complimentary colors, analogous colors and so on. Painter's Picker requires 10.2, 10.3 or 10.4 running on a G3, G4, G5 or Intel Mac (version 2.1 is universal). A free trial version is available, and a single license will run you $15.85US.
[Via Daring Fireball]