Massachusetts Institute of Technology
Mit: Useful Trigonometric Identities
This list of trigonometric identities includes the factoring formulas near the bottom. A great tool for reference.
Clark University
Clark University: Dave's Short Trig Course
This Clark University trigonometry tutorial site features questions and answers about what students can learn from trigonometry.
Lawrence Hall of Science
The Math Page: Trigonometry: Trignometry of Right Triangles
Here is a site that clearly and thoroughly explains the trigonometry of a right triangle, including finding the values of all six trig functions given a single value. There are example problems solved, problems for the student to...
Math Words
Math Words: Cofunction Identities
This MathWords.com site, which is a glossary of math terms, shows the relationship in radians and degrees between sin and cos, tan and cot, and sec and csc.
Other
Technical Tutoring: Trigonometry: The Very Fast Review
This site is a quick review of the functions and identities in trigonometry. Very helpful site.