Nope. Everything on Khan Academy's site is completely free to use and plans to stay that way. We do not require contracts, and have no spam and no ads. Instead, the organization is supported through the generosity and donor of grants. That way, you can focus on learning.
So what are you waiting for? Start learning!
Khan Academy is a 501(c)3 non-profit on a mission to provide a FREE world-class education for anyone, anywhere.