Emulino - an emulator for the Arduino platform by Greg Hewgill
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.
SimulAVR - The SimulAVR program is a simulator for the Atmel AVR family of microcontrollers.
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
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.
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.