Information adapted from Fiske Guide to Getting Into the Right College.
Occasionally, students may want to send along an extra recommendation beyond those requested in the application. Though the first impulse should always be to send the one or two best recommendations, an extra one from someone who knows you well may not hurt, especially if that person can highlight a different facet of your record or personality than the others.
As a rule, you should not exceed the number of recommendations requested by more than one. (The waitlist is an exception to this rule.)