Kaggle

Kaggle is the largest data science community in the world. At any given time, numerous active competitions can be found, usually with prize money that can be quite substantial. There are also introductory-level competitions.

DrivenData

DrivenData organizes online data science competitions where the data and the problem are proposed by a socially-minded organization. Similar to Kaggle, these events generally have cash prizes, often substantial, and there are also educational competitions.