Wolfram Research
Wolfram Mathworld: Prime Factorization
This site gives a wonderful explanation of prime factorization including a very helpful table in understanding prime decomposition. Look further down the page to get information on Gaussian primes.
Amby Duncan-Carr
Prime Factorization: Amby's Resources
What is prime factorization? This site goes through a step-by step process explaining different ways to find the prime factorization.