Open source alternatives for Matlab?

Go with Octave, it is really decent nowadays.

If you need any help with the LTSpice2Matlab() function shoot me a PM. The orignal one was incompatible with Octave, but I managed to find/modify a version that is working reasonably well. I use it regularily.
 
LTSpice2Matlab() is a function that is able to read LTSpice .raw files and show the SPICE results into a array in your workspace.

If you use LTSpice as a commandline tool it is possible to do some, even more, sophisticated analysises than LTSpice features in standalone mode.

Sometimes I just use it for convinient post-processing.
 
Octave works, although it's single threaded for the most part which means for big parallel jobs it can be a little slower. Have a look at the octave forge as it pretty much has most of the Matlab libraries.

Currently thinking of writing a octave neural network to solve the best set of resistors for an attenuation network.