Open Source Maths Package Wanted

I am looking for the best of the Open Source maths packages. I've looked at
OSDN Freshmeat and there are hundreds of packages too many to go through.

http://www.freshmeat.net

Basically I am reading Roger Penrose's "The road to reality" and want to be
able to play with formulae visualizing and doing symbolic manipulation.

Many thanks in advance,

Aaron



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