Introduction to Differential Geometry: http://people.hofstra.edu/faculty/Stefan_Waner/diff_geom/tc.html

Book on Differential Geometry: http://www.topology.org/tex/conc/dg.html

Mathworld on Differential Geometry: http://mathworld.wolfram.com/topics/DifferentialGeometry.html

Cartan Geometry on Wikipedia: http://en.wikipedia.org/wiki/Cartan_connection

Books on Cartan and Differential Geometry

Cartan for Beginners: Differential Geometry Via Moving Frames and Exterior Differential Systems

Elementary Differential Geometry by Andrew Pressley

Differential Geometry by Erwin Kreyszig

Differential Geometry of Curves and Surfaces by Manfredo do Carmo

Amazon Books on Differential Geometry

Amazon Books on Cartan Geometry

Links on General Relativity

Scienceworld on General Relativity: http://scienceworld.wolfram.com/physics/topics/GeneralRelativity.html

Tutorial on General Relativity: http://math.ucr.edu/home/baez/gr/

Lecture Notes by Sean Carrol of MIT on General Relativity: http://pancake.uchicago.edu/~carroll/notes/

Books on General Relativity

A First Course in General Relativity by Bernard Schutz

General Relativity by Robert Wald

Advanced General Relativity (Cambridge Monographs on Mathematical Physics)

Amazon Books on General Relativity