16. Solving Einstein's Equations in Three Dimensions
This is a Mathematica notebook showing how to plot light cones in two plus one spacetime dimensions with a point mass.
http://www.theory.caltech.edu/people/patricia/test/Einstein3.html#Einstein31

18. Southampton GR Explorer
An introduction to Einstein's theory of General Relativity and related topics. These pages include informative text, pictures and movies.
http://www.maths.soton.ac.uk/relativity/GRExplorer/

19. The Cosmological Constant
An overview of why Einstein added an extra term in general relativity, and why it is still examined.
http://pancake.uchicago.edu/~carroll/encyc/