Prime numbers are used in cryptography. The RSA cryptography system consists of two large prime numbers chosen by the user. The enciphering key is the product of these two primes. Because it is extremely difficult and time consuming to factor large numbers, it is practically impossible to find the deciphering key.
Banks also use prime numbers to provide security for ATM machines, etc. List of the first 10,000 prime numbers (University of Utah)
The Prime Machine [JAVA] at the University of Utah The applet on this page lets you explore the set of prime numbers.
