Real parts
Batteries, transistors, op-amps, relays, regulators and a 555.
Circuit Lab
Place parts on a schematic and wire them, and a real SPICE solver runs it: waveforms, per-part readings and warnings.
It opens with sample data already in it. No sign-up.
Mechanisms too
DC motors, servos, steppers, gears and an Arduino Uno running your sketch.
A real solver
ngspice runs the transient and reports what each part did.
