Clark University
Clark University: Euclid's Elements
A review of the 13 books of Euclid's "Elements" text can be found here. Each of the chapters are broken down into definitions, propositions, and common notations that are used throughout this overview.
Other
Mrao: Imaginary Numbers Are Not Real
This site presents a tutorial introduction to the ideas of geometric algebra. Gives an outline of geometric algebra and discusses the geometric product, imaginary numbers, and more.