Project Euler members will be glad to know that it has now recovered from the June 20 security breach, and users can again access their accounts. (Be prepared to change your password on the initial login.) For those not familiar with Project Euler, it provides:

a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

The site is language-neutral, but I have found Perl to be an excellent language for tackling the problems; and, of course, the challenges are a useful way to practice one’s Perl skills.