Tenzin suggests...
"Project Euler"
http://projecteuler.net/
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Synopsis: What is Project Euler?"
Project Euler is 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 motivation for starting Project Euler, and its continuation, is to
provide a platform for the inquiring mind to delve into unfamiliar
areas and learn new concepts in a fun and recreational context.
This is also on our website!