GNU Octave - GNU Octave is a programming language for scientific computing.
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
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.
Xcos - Xcos is a graphical editor to design hybrid dynamical systems models.
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.