Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
Logisim - Logisim is an educational tool for designing and simulating digital logic circuits.
simavr - a lean and mean Atmel AVR simulator for linux
BOOLR - Open-source digital logic simulator
Emulino - an emulator for the Arduino platform by Greg Hewgill
Logicly - - Design circuits quickly and easily with a modern and intuitive user interface with drag-and-drop...