Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
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.
simavr - a lean and mean Atmel AVR simulator for linux
Proteus PCB design - Proteus PCB design combines the schematic capture and ARES PCB layout programs to provide a...
Emulino - an emulator for the Arduino platform by Greg Hewgill
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