Hydrogen peroxide, sometimes used as a bleaching agent for hair. However, if proper preparation, you can easily use this easily available chemical to whiten your teeth.
When preparing, make sure the peroxide is diluted with at least 1 part water to every 1 part peroxide and be very careful not to swallow the solution, as it could lead to blindness and even death.
While it can taste pretty horrible, there are people who swear by it. Try it and see if it works.