Friction Labs is a group of long-time climbers and scientists who realized that all the chalk out there was just awful. It just didn't make sense that we were obsessing over the rubber and fit of our shoes but totally ignoring the quality of the stuff we put on our hands for grip. They decided to use science to engineer the perfect chalk for rock climbing. They are committed to doing everything they can to give you the best grip possible in order to help you climb better and send more problems.
My 3 Favorites are:
Check out Frictionlabs.com while you're at it!