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.
Oregano - oregano - An electrical engineering tool for GNOME
BlackBoard Circuit Designer - Help building prototypes on so called breadboards easily
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
PCB Wizard - PCB Wizard 3 is a powerful package for designing single-sided and double-sided PCBs.