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.
Other
All Elementary Mathematics: Trigonometric Equations
This site gives a definition for what a trigonometric equation is, gives a listing of the simplest trigonometric equations, and lists seven main methods of solution of trigonometric equations with examples.
Wolfram Research
Wolfram Math World: Half Angle Formulas
This page, from Eric Weisstein's comprehensive Math World site, offers information on half-angle formulas. The site includes traditional trigonometric fuctions half-angle formulas as well as hyperbolic half-angle formulas. Includes links...