Prime Numbers:

http://factmonster.com/ipka/A0876084.html
   Explains prime numbers and even gives a table of primes to 1,000.

http://mathforum.org/dr.math/faq/faq.prime.num.html
   Explains prime numbers and composite numbers. 
   It will tell you about the largest known prime number. 
   Write it down and give it to me for extra credit!

How to Factor (GCF/LCM)

http://www.mathdork.com/lessons.html
How to factor.  Click on Factoring then click on "view free"

http://www.math.com/school/subject1/lessons/S1U3L1GL.html
   What are factors and multiples?  Step by step explanation including some practice.

http://mathforum.org/dr.math/faq/faq.learn.factor.html
   Discusses how to factor GCF, LCM, LCD

http://mathforum.org/library/drmath/view/58140.html
   3 methods how to find LCD

http://mathforum.org/library/drmath/view/58527.html
   How to find LCD, LCM, GCF

http://mathforum.org/library/drmath/view/58569.html
   How to find LCM

http://www.math.com/school/subject1/lessons/S1U3L2GL.html
   What is GCF?  Explanation and practice.

http://www.math.com/school/subject1/lessons/S1U3L3GL.html
   What is LCM? Explanation and practice.