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.
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
Ngspice - Ngspice is a mixed-level/mixed-signal circuit simulator.
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical 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.
Oregano - oregano - An electrical engineering tool for GNOME