Octave

Octave is pretty close to matlab which is good enough for my problem of plotting the FFT of a signal and the frequency response of a filter.

No doubt matlab is a well known standard tool, but for Octave users it means support in scripting and usage is not longer away than the closest matlab acquainted person.

Plotting the frequency response of a HP FIR filter is this hard:

>> a = []
>> b = [ coefficients ]
>> freqz(b,a)

Octave solved my problem, plotting went well and looked 'scientific' enough and copying plots to clipboard and then to word was painless. Octave is now added to my arsenal of weapons.

As an end note: Octave don't support image formats very well out of the box, so you will need Biosig in order to do that.