Emulino - an emulator for the Arduino platform by Greg Hewgill
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
simavr - a lean and mean Atmel AVR simulator for linux
SimulAVR - The SimulAVR program is a simulator for the Atmel AVR family of microcontrollers.
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.
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