Fathom – http://www.fathom.com
If you’re looking for a way to augment your learning, or just for some personal interest reading, the Fathom directory might be just the place to begin. Columbia University, partnering with museums, libraries, other educational institutions, and more the world over, provide a wealth of free online learning materials to suit every taste. Lectures, articles, seminars, and more, cover a variety of subjects from biodiversity, to Shakespeare, to Egyptology.
Started in 2000 with a goal of “providing high quality educational resources to a global audience through the Internet”, Fathom has grown to a network of cultural institutions and educational facilities that share in providing content for the site. Though Fathom is no longer in development, their complete catalogue of resources remains available at their website for free use by anyone who is interested.
Equation Sheet – http://www.equationsheet.com/
More than just a reference site, Equation Sheet provides visual examples of mathematical equations of all types, from simple algebra to the most complex geometry. Also covered are chemistry, physics, statistics, trigonometry, and much more.
They also provide information on SI units, conversion tools, symbols, and also provide a list of software to aid you in your numeric quests.