Emulino - an emulator for the Arduino platform by Greg Hewgill
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
SimulAVR - The SimulAVR program is a simulator for the Atmel AVR family of microcontrollers.
Fritzing - Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to...
Xyce - Xyce is an open source, SPICE-compatible, high-performance analog circuit simulator, capable of solving extremely large circuit problems by supporting large-scale parallel computing platforms.
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.