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.

Aspen Plus - Maximize profits using a plant-wide simulation solution that combines unparalleled accuracy and engineering collaboration with time-saving workflows.

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

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

SMath Studio - Tiny, powerful, free mathematical program with WYSIWYG editor and complete units of measurements support.