Textbook website -- Contains full solutions to odd-numbered problems, as well as some applets and links to websites about specific topics covered in the text

The Prime Pages -- A font of information about prime number research, records and resources

GIMPS -- The Great Internet Mersenne Prime Search: find out how to lend your computer to the search for the next largest prime number!

Historical Cryptography -- Information about historical ciphers

MacTutor -- The history of mathematics archive hosted by St. Andrews University, a great source of historical and biographical information about mathematics

Sieve of Eratosthenes -- A Java applet for prime number sieving

How to Write Proofs -- A nice primer on proof-writing by Larry W. Cusick at CSU Fresno