KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
Proteus PCB design - Proteus PCB design combines the schematic capture and ARES PCB layout programs to provide a...
simavr - a lean and mean Atmel AVR simulator for linux
Altium Designer - PCB Design Software
Pspice - OrCAD PSpice technology provides the best, high-performance circuit simulation to analyze and refine your circuits, components, and parameters before committing to layout and fabrication