Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
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
simavr - a lean and mean Atmel AVR simulator for linux
Emulino - an emulator for the Arduino platform by Greg Hewgill
PCBWeb - PCBWeb is a 100% free Windows desktop CAD application for designing and manufacturing electronics...
BlackBoard Circuit Designer - Help building prototypes on so called breadboards easily