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.
Other
Technical Tutoring: Trigonometry: The Very Fast Review
This site is a quick review of the functions and identities in trigonometry. Very helpful site.