My younger cousin (2nd grade USA) is advancing very quickly in his current math class. The school does not have much to support him in moving ahead in the material. What websites (preferably game or challenge based) are good for kids that want to practice and learn more math?
Britain has the NRich site, which has some great targeted problems, games and interactivities:
You'll be looking for the Upper Primary or Lower Secondary level.
I would propose Kenken (http://www.kenkenpuzzle.com). It gives you a feeling of numbers, that is: Which (prime) factors appear in which numbers.
This was a popular comment and is more appropriate as an answer.
I think the games on the Freudenthal Instituut website are very good in many ways, especially for discovery/inquiry learning. It's always good to follow up with some formalisation of the ideas afterwards. http://www.fisme.science.uu.nl/publicaties/subsets/en for the website.
From what I know, Alexey Root has a series of wonderful chess books that aren't bad! Yamie Chess also seems to be popular although I haven't touched any of its contents. It was invented by former US Women's Champion Jennifer Shahade.