Calspex - Calibration
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.
GageList - Simple online calibration software
Circuit Simulator - Animated electronic circuit simulator using ideal components to visualize voltage and current.
QMSys GUM - Calibration
Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.