Facial Expressions

Based on Paul Ekman's research, it appears there are 6 basic, innate and universal expressions.  All others are combinations of these, in varying degrees of intensity.  For more info, see Scott McCloud's Making Comics (from which this info is unabashedly ripped) and for an amazingly in-depth view of the subject, Gary Faigin's Complete Guide to Facial Expression.  As always, this info on its own is not enough - always try to make the expression with your own face and inhabit the emotion you are trying to draw - try to draw the feeling not just the surface.