Logisim - Logisim is an educational tool for designing and simulating digital logic circuits.
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
BOOLR - Open-source digital logic simulator
Proteus PCB design - Proteus PCB design combines the schematic capture and ARES PCB layout programs to provide a...
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.