http://hpc.sourceforge.net/ lists a number of useful packages. This is a partial description:
If you want to use your G4/G5 with Mac OS X for some good numerical work .. this could be useful to you. I have here, binaries, source, documentation and instructions to install Fortran, MPI, OpenMP, Octave, Cactus, Globus, RNPL, GIMPS, Computing Libraries, Pascal, LISP, GRAVSIM, FEYNMAN, etc. on Darwin OS.
Speaking from personal experience, Octave is really, really cool. I’m rather fond of NumericalComputation also.
There is an octave wiki page at http://omni.isr.ist.utl.pt/~etienne/cgi-bin-etienne/wiki.pl , with the beginnings of a MacOSX section http://omni.isr.ist.utl.pt/~etienne/cgi-bin-etienne/wiki.pl?OctaveForMac
There are still parts of octave/octave-forge that need porting.
Eigenmath: http://eigenhead.com/ is a nice little symbolic math tool. Seems to be easy to use also!
http://www.netlib.org/ (the Netlib repository of mathematical software, etc.)
For HandlingLargeNumbers see that page