Visible Euclidean Algorithm - http://www.math.umn.edu/~garrett/crypto/a01/Euclid.html Enter two positive integers, and click the button to see the operations of the Euclidean Algorithm.
Future Directions in Algorithmic Number Theory - http://aimath.org/WWN/primesinp/ Some of the conjectures and open problems motivated by the "PRIMES is in P" papers, compiled at the AIM.
Computational Number Theory - http://userpages.umbc.edu/~rcampbel/NumbThy/Class/ Lecture notes by Robert Campbell, December 1997.
Basics of Computational Number Theory - http://userpages.umbc.edu/~rcampbel/NumbThy/Class/BasicNumbThy.html Notes and Javascript illustrations by Robert Campbell.
The LLL Algorithm - http://www.dice.ucl.ac.be/~fkoeune/LLL.html Papers on the LLL algorithm and its applications collected by François Koeune.
Computational Projects in Number Theory - http://www.ieeta.pt/~tos/hobbies.html Interesting problems, usually requiring extensive verifications or enumerations, to occupy the idle CPU time of workstations and personal computers. Compiled by Tomás Oliveira e Silva.