Purpose: Perform GR calculations (in components, not symbolically).
Installation: Save GREATER2.m on your computer in your Mathematica apps directory. If you don't know where this is, type $Path in Mathematica and save it in one of the directories that is listed.
Next, download the example notebook to learn the basic features and usage of GREATER2. This does not include all the features, you can read the code to find the others.
Features: Easy metric input (from line element); covariant and lie derivatives; unit normals; wedge products; Cotton tensor calculation; D'Alembertian; easily change coordinate systems; various other things. With minimal effort I've also used this package to do more complicated things like compute the Brown-York stress tensor on a hypersurface, find the second order equations of motion for topologically massive gravity, etc.
Much of the code is based on Matt Headrick's diffgeo package and Tristan Hubsch's GREAT.m.
Development/Bugs: Please email me if you find a bug. If you would like to edit the code, you'll need the notebook file GREATER2.nb. This package was written for Mathematica 8, so it does not use the built-in tensor functions that appeared in version 9.