Other
Interactive Algebra: Factoring With Negatives
These randomly generated polynomials allow you practice factoring. Some of the polynomials contain leading coefficients that are negative. Hints are available when needed and, when ready, you can check your answer.
Other
Interactive Algebra: Factoring, Lesson 2 2
Practice factoring trinomials with these randomly generated problems. Stuck on finding the right factors? Hints are available, as well as the answer. Continue practicing until you feel comfortable factoring.
National Council of Teachers of Mathematics
The Math Forum: Understanding Algebraic Factoring
This site from the Drexel University explains the principles of factoring symbolically using algebra tiles.