KiCad - A Cross Platform and Open Source Electronics Design Automation Suite

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.

Fritzing - Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to...

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.

Virtual Breadboard - It is a circuit simulator on a virtual breadboard.

Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.