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