Open source lecture notes and textbooks

The course Measure Theory by D.H.Fremlin includes TeX source.

Topology Course by Aisling McCluskey and Brian McMaster in HTML.

Diverse lecture notes by Conor Houghton.

Cryptography homework by Boaz Barak.

Digital Image Processing.

Abstract Algebra handouts and Number Theory lecture notes.

Many of the courses in MIT's OCW have such notes: http://ocw.mit.edu/courses/find-by-department/

Here are a few I've found over the years. I've included some brief comments on the first four, which I've personally used in my classes.

• Stitz and Zeager Precalculus materials
  Outstanding. Includes a lot of ancillaries like answers and youtube videos. CCL
• Mooculus
  Very, very good calculus MOOC offered at Ohio State University. Text is freely available by itself. One limitation - it covers only single variable. CCL
• Whitman Calclus by David Guichard
  Very good and complete Calculus text. CCL
• APEX Calculus
  The most recent open text that I've used for Calculus. Almost as good as Mooculus and almost as complete as Guichard.
  CCL
• A First Course in Complex Analysis Beck, Marchesi, and Pixton
  Very good - Freely available but not open.
• Linear Algebra by Jim Hefferon
  This book won the 2020 Daniel Solow Author's Award from the Mathematical Association of America.
  GFDL or CC BY-SA
• A First Course in Linear Algebra GFDL
• Abstract Algebra by Tom Judson GFDL
• Basic Analysis - An Introduction to Real Analysis by Jiri Lebl CCL