Contests, problemsets and challenges
Not for a long time, I have been detected online judge's, which give's problem. Your mission is to find solution in C++/Java/Pascal, and online judje will check it with multiple tests. Tests are hidden, you can't view it, but system give's you 1 test, and problem description. Most of problems are difficult. Main thing is algorithm, not a programming technique.
http://acm.timus.ru - Russian Online Judge. Interface is multilanguage.
http://uva.onlinejudge.org - Nice interface with statistics.
If you need help, I will give you wiki pages, where you can find hints & tricks.
http://www.comp.nus.edu.sg/~stevenha/index.html
http://www.algorithmist.com/index.php/
eyeOS Application Development Contest
Every one can participate in this programming contest. I'm registered there. And my mission is to create 3 applications under eyeOS. Registration started in June 3rd, ends at July 1st
Here are prizes.
* Winner: A Dell Mini 10v with Ubuntu (Intel®Atom®Processor Z520, 1GB RAM, 10.1" Widescreen Display, Intel GMA500) + An eyeOS Shirt of your size
* 1st Runner-up: A Network Storage Hard Drive (LaCie Network Space, 1TB) + An eyeOS Shirt of your size
* 2nd Runner-up: A 20" Dell flat panel (S2009W 20" HD Widescreen Monitor, 1600x900, 5MS response) + An eyeOS Shirt of your size
EyeOS development is non standart web development 
I'm suggesting you to read this pdf minibook before making any application.
There are GUI maker application under eyeOS, very useful for developers!
Documentation and registration here! Hurry up! 