Software

The Cactus Code
A modular manageable high-performance 3D tool for Numerical Relativity.
http://www.cactuscode.org/

CarpetCode
A mesh refinement driver for the Cactus code; the primary application is the simulation of black hole spacetimes.
http://www.carpetcode.org/

Cartan: A Mathematica Package for Tensor Analysis
Product information from Ad Infinitum AS about a package for use with the Mathematica software; allows interactive tensor calculations in spacetimes of arbitrary dimension.
http://adinfinitum.no/cartan/

Form Symbolic Manipulation System
A free software package for manipulating tensors (abstract index notation) and other multi-component objects; highly suitable for calculations in general relativity and related theories such as supergravity. Documentation, course material, and possibilities for download.
http://www.nikhef.nl/~form/

GRAVLENS
Software for modelling gravitational lens systems; created and maintained by Chuck Keeton at Rutgers University.
http://redfive.rutgers.edu/~keeton/gravlens/

GRTensorII
A computer algebra package for performing differential geometry calculations. Demonstrations of the package's applications to general relativity.
http://grtensor.org/

GRTensorM
A version of GRTensor II for use with the Mathematica software.
http://www.astro.queensu.ca/~grtensor/grtmma.html

Kranc
A suite of Mathematica packages for numerical relativists, which helps with tasks such as performing 3+1 decompositions, analytical manipulations of tensorial systems of equations (such as change of variables), computation of characteristics or linearizations, and automatic code generation.
http://numrel.aei.mpg.de/Research/Kranc/

LORENE (Langage Objet pour la RElativite NumeriquE)
Provides a set of C++ classes to solve various problems arising in numerical relativity and other areas of computational astrophysics.
http://www.lorene.obspm.fr/

openGR
An open framework for large numerical simulations in general relativity.
http://wwwrel.ph.utexas.edu/openGR/

Ricci
A Mathematica package for doing tensor calculations in differential geometry and general relativity.
http://www.math.washington.edu/~lee/Ricci/

Riemann Package for Maple
Package written in Maple for calculating tensor components in General Relativity and performing some abstract tensor manipulation; suitable for calculations in general relativity and for quantum field theory in curved spacetime. Features brief description, download for Maple V and for Maple 6 or higher, and examples.
http://www.cbpf.br/%7Eportugal/Riemann.html

Whisky
A code to evolve the equations of hydrodynamics in curved spacetime (relativistic numerical hydrodynamics).
http://www.whiskycode.org/