Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
simavr - a lean and mean Atmel AVR simulator for linux
Fritzing - Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to...
Emulino - an emulator for the Arduino platform by Greg Hewgill
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.