QUCS - Qucs, briefly for Quite Universal Circuit Simulator, is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit.

MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming

LTspice - LTspice® is a high performance SPICE simulation software, schematic capture and waveform viewer with enhancements and models for easing the simulation of analog circuits.

Wolfram Mathematica - Mathematica has characterized the cutting edge in specialized processing—and gave the chief calculation environment to a large number of pioneers, instructors, understudies, and others around the globe.

PyLab_Works - PyLab_Works is a free and open source replacement for LabView + MatLab, written in pure Python.

Maxima - Maxima is a fairly complete computer algebra system written in Lisp with an emphasis on symbolic computation.