simavr - a lean and mean Atmel AVR simulator for linux

Yenka - Yenka is a new generation of educational modelling tools from Crocodile Clips.

Pspice - OrCAD PSpice technology provides the best, high-performance circuit simulation to analyze and refine your circuits, components, and parameters before committing to layout and fabrication

Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.

Xyce - Xyce is an open source, SPICE-compatible, high-performance analog circuit simulator, capable of solving extremely large circuit problems by supporting large-scale parallel computing platforms.

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.