Khan Academy is a collection of video tutorials that are completely free to watch. There is no need to create a login but doing so allows you to save where you left off and earn rewards, such as badges. It includes is a collection of math lessons that start from the basics.
Founded in 2012 by computer science professors from Stanford University, Coursera now includes content from over 150 partners (Yale, Johns Hopkins, University of Edinburgh, etc.). Math and logic courses are available, but also courses on writing and publishing in the sciences.