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I love sites like these! I wish there was one for every language and every learning subject.

Take a look at http://projecteuler.net/problems It's a wonderful collection of non language specific mathematical and/or computer programming problems.

Python challenge is interesting too, but not a good intro to the language.

http://www.pythonchallenge.com/

upon a cursory look I couldn't find a decent version for ruby, but there are many out there that do it differently.

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