Cadabra is a computer algebra system designed specifically for the solution of problems encountered in field theory. It has extensive functionality for tensor polynomial simplification including multi-term symmetries, fermions and anti-commuting variables, Clifford algebras and Fierz transformations, implicit coordinate dependence, multiple index types and many more. The input format is a subset of TeX. Both a command-line and a graphical interface are available.
Key features of Cadabra:
Technical highlights:

License: GNU GPL.
Tags: algebra, computer algebra system, physics.
Interfaces: command line, X.
Source language: C++.
Maintainer: Kasper Peeters.
Developer: Kasper Peeters.
Homepage: http://www.aei.mpg.de/~peekas/cadabra/.
Documentation: http://www.aei.mpg.de/~peekas/cadabra/documentation.html.
Support: http://www.hepforge.org/lists/listinfo/cadabra-discuss.