Logisim - Logisim is an educational tool for designing and simulating digital logic circuits.

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.

Logicly - - Design circuits quickly and easily with a modern and intuitive user interface with drag-and-drop...

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.

Digital - A simulator for digital circuits.

PyLab_Works - PyLab_Works is a free and open source replacement for LabView + MatLab, written in pure Python.