Project Euler
Most of you would probably know about this, but anyways.
Project Euler is a site with a large set of mathematical and programming problems. And they are no way a piece of cake, cause they make you think, think hard. 
Here’s their “About” for additional information:
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.
What’re you waiting for, let those knuckles crack!
Here’s my profile page at Project Euler.
Thanks to Sykora for introducing that to me.
