Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
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.
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.
Circuit Solver - Circuit Solver uses a simple drag and drop approach to building circuits.
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
ECStudio - ECStudio app provides you with features that allow you to learn about the building of electric circuits and electronic circuit simulation so you can calculate various values of circuits right at your fingertips.