[pm90]

It depends on what you mean by "a serious alternative". DSP functions exist in python too (http://docs.scipy.org/doc/scipy/reference/signal.html), but where MATLAB wins out is with their Simulink library, which doesn't have an open source alternative.....yet. AFAIK.

[shared4you]

There is Scicos [1] which is the "Simulink" of Scilab, itself a Matlab-lookalike.

[1]: http://www.scicos.org/