Free textbooks in geometry
Combinatorial Geometry
 The Geometry Junkyard Usenet clippings, web pointers, lecture notes, research excerpts, papers, abstracts, programs, problems, and other stuff (by Prof. David Eppstein)
 Geometry in Action by Prof. David Eppstein.
Differential Geometry

Introduction to Tensor Calculus and Continuum Mechanics, by John H. Heinbockel
 Topics in Differential Geometry, Peter W. Michor.
 Intrinsic Geometry of Surfaces by A. D. Aleksandrov and V. A. Zalgaller
 Riemann Surfaces, Dynamics and Geometry Curtis McMullen
 Invariance Theory, the Heat Equation, and the AtiyahSinger Index Theorem by Peter B. Gilkey
 Differentiable Manifolds lecture notes (growing) by Mariusz Wodzicki
 Lecture notes for the course in Differential Geometry by Sergei Yakovenko
 Differential Geometry lecture notes by Gabriel Lugo
 Differential geometry reconstructed by Alan U. Kennington
 Course of Differential Geometry by Ruslan Sharipov
 A Modern Course on Curves and Surfaces by Richard S. Palais
 Twistor Geometry and Gauge Theory by Martin Wolf
 Complex Geometry of Nature and General Relativity by Giampiero Esposito