Scilab
http://www.scilab.org/It is like Matlab, but completely free.
Here is list of features from official web site:
# 2-D and 3-D graphics, animation
# Linear algebra, sparse matrices
# Polynomials and rational functions
# Simulation: ODE solver and DAE solver
# Scicos: a hybrid dynamic systems modeler and simulator
# Classic and robust control, LMI optimization
# Differentiable and non-differentiable optimization
# Signal processing
# Metanet: graphs and networks
# Parallel Scilab using PVM
# Statistics
# Interface with Computer Algebra: Maple package for Scilab code generation
# Interface with Tcl/Tk