I can't believe that Python and its vast scientific application libraries are not mentioned in this thread. Let me be little bit bias and mention projects like [PyDSTool][1]. What makes Maple or Mathematica language simpler than Python? Similarly time proven but proprietary computer algebra system MuPad now a part of MATLAB is not mentioned. FreeMat, the cleanest (other two being GNU Octabe and Scilab) reimplementation of MATLAB API, is also not mentioned. 

P.S. I was surprised that people even mentioned things like Maple and Mathematica. I personally have not meet a person who had a pick at the source code of these two systems.
However, I have friends who have worked for MathWorks and have seen the source code of MATLAB.

  [1]: http://www2.gsu.edu/~matrhc/PyDSTool.htm