QUCS - Qucs, briefly for Quite Universal Circuit Simulator, is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit.
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
Fritzing - Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to...
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.
Autodesk EAGLE - Autodesk EAGLE is an electronic design automation (EDA) software.