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
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
EasyEDA - EasyEDA - Web-based EDA suite; runs in browser.