Scientific Computing
After running into unsurmountable (for me) problems in installing Matlab R13 on Linux, I did some research for a Matlab replacement. This guy compares Octave (GNU), Scilab (INRIA) and python, and there’s also the R Project for statistics, and bc (GNU) in a terminal for quick and simple calculations, with syntax somewhat similar to C. Let’s not forget Sage.

octave and scilab are available in the repositories (octave in universe/math, scilab in multiverse/math).
according to the instructions at http://cran.r-project.org/bin/linux/ubuntu/, i did:
echo "deb http://rm.mirror.garr.it/mirrors/CRAN/bin/linux/ubuntu hardy/ #R-project" | sudo tee -a /etc/apt/sources.listgpg --keyserver subkeys.pgp.net --recv-key E2A11821
gpg -a --export E2A11821 | sudo apt-key add -
sudo aptitude update
sudo aptitude install r-base