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
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
Oregano - oregano - An electrical engineering tool for GNOME
Emulino - an emulator for the Arduino platform by Greg Hewgill
BlackBoard Circuit Designer - Help building prototypes on so called breadboards easily