This is a very useful site. You enter any positive integer and it will return a prime factorization of that integer.
The Prime Pages; there is a lot here. Articles include the “Guinness book of” prime number records, how to find primes, conjectures and open problems, and some special types of primes:
This is a nice site, but is probably more than you want to know about primes:
Listen to the primes:
The largest known primes:
A nice reference for perfect numbers is at this site:
There is a lot of interesting material on the web concerning Goldbach’s conjecture. The follow site provides a numerical calculator.
This is a fun site that combines the ideas of divisibility and patterns on Pascal’s Triangle (which was introduced in Chapter 1). If this site does not open for you, you may need to install a Java plugin.