• AllAboutCircuits.com
• Principles of Semiconductor Devices, Bart Van Zeghbroeck @ University of Colorado
• A Review of Electronics, J. B. Calvert
• Wikibooks is the explicitly pedagogical grandson of Wikipedia
• more
• [MassiveBodyOfFacts]
  • Microwaves101.com (wait... waveguides are actually used IRL?)
  • Hyperphysics (pretty broad, but has a lot of useful detail in the electromagnetism section for EE)

DSP

• Julius Orion Smith III
  • Mathematics of the Discrete Fourier Transform (DFT) with Audio Applications
  • Introduction to Digital Filters with Audio Applications
  • physical Audio Signal Processing for Virtual Musical Instruments and Audio Effects
  • Spectral Audio Signal Processing
• The Scientist and Engineer's Guide to Digital Signal Processing, by Steven W. Smith, Ph.D.

Maths

• more

Organizations dedicated to creating open textbooks

• CK12 (homepage, wikipedia)
• Wikimedia
  • Wikibooks (homepage, wikipedia)
  • Wikiversity (homepage, wikipedia)

Open-textbook indexing websites

• E-BooksDirectory.com