I used to be a member of +Ma's Reversing, and later became a member of Caesum's Electrica. Recently I've played Bright Shadows. Are there other good sites for a challenge?
Question reopened; not a duplicate. Similar, yes, but slightly different.
Related: http://stackoverflow.com/questions/24692/where-can-you-find-funeducational-programming-challenges
xkcd turned me on to Project Euler, which can be completed in any language. It's pretty challenging after a while (hope you enjoy recursion!) and a bunch of people are doing it so there's a good amount of comparing solutions in just about every language under the sun.
Duplicate --- http://stackoverflow.com/questions/24692/where-can-you-find-funeducational-programming-challenges
Sneak peek (links are inside that answer)
You might want to add your own links there, as you seem to have some that aren't on the list already
projecteuler.net is good if you like mathy problems Python Challenge if you like riddles and python
I enjoy Project Euler (math-ey problems) and this contest site (ICPC type problems)
I've come to like Sphere Online Judge. The variety of languages is really nice, although they don't have dc installed. The maintainers also very actively answer questions on the forums, even if it's just, "Why isn't my program working?" I'm not sure how they find time.
You can try my website http://www.cstutoringcenter.com/problems for some "warm up" questions before going on to Project Euler or Python Challenge. Register free to submit answers.
This one is my personal favorite, full of real world problems varying in topic, and the site has a great supporting community to boot:
Not exactly in the same vein, but codeeval.com allows you to create your own programming challenges and invite people to solve them. Solutions can be accepted and auto evaluated in several programming languages such as C/C++/Java/Python/Ruby etc...
www.codeeval.com