Online books
General
 Roger D. Blandford (Stanford) and Kip S. Thorne (Caltech), Applications of classical physics
Classical Mechanics
 L.D. Landau and E.M. Lifshitz, Mechanics (Course of Theoretical Physics, Volume 1)

David Morin (Harvard),
Introduction to Classical Mechanics With Problems and Solutions
Sample chapters, errata, Google books link, additional chapter
 David Tong (Cambridge), Lectures on Classical Dynamics
 Richard Fitzpatrick (University of Texas at Austin), Analytical Classical Mechanics
 John Baez (University of California, Riverside), Classical Mechanics
 Alain J. Brizard (Saint Michael's College), An introduction to Lagrangian mechanics
 Simon J. A. Malham (HeriotWatt University), An introduction to Lagrangian and Hamiltonian mechanics
Continuum Mechanics
 L.D. Landau and E.M. Lifshitz, Theory of Elasticity
 L.D. Landau and E.M. Lifshitz, Fluid Mechanics
 Simon J. A. Malham (HeriotWatt University), Introductory fluid mechanics
 Allan F. Bower (Brown University), Applied Mechanics of Solids
 Horace Lamb, Hydrodynamics
 A. E. H. Love, A treatise on the mathematical theory of elasticity
Scientific Computing:
 Cleve Moler (MathWorks), Numerical Computing with MATLAB
 Morten HjorthJensen (University of Oslo), Computational Physics
 Eric Ayars (California State University, Chico), Computational Physics with Python
Mathematics:
 James Nearing, Mathematical Tools for Physics, Dover Publications, 2010.
 Michael Stone and Paul Goldbart, Mathematics for Physics (also here), Cambridge University Press, 2009.
 Herbert Wilf, Mathematics for the Physical Sciences, Dover Publications, 2006.
 Michael P. Brenner, Physical Mathematics, Harvard AM201, 2010.
 Gilbert Strang, Calculus Online Textbook, MIT OpenCourseWare, 2005.