Digital - A simulator for digital circuits.
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
BOOLR - Open-source digital logic simulator
Emulino - an emulator for the Arduino platform by Greg Hewgill