University of Waterloo (Canada)
Wired Math: Primes and Prime Factors [Pdf]
Explore primes, prime factors, and composites using this collection of worksheets. Lots of detailed examples, interesting math facts, and colorful visuals. Acrobat Reader required.
Quia
Ixl Learning: Prime Factorization
Brush up on your math skills relating to prime factorization then try some practice problems to test your understanding.