Logisim - Logisim is an educational tool for designing and simulating digital logic circuits.
Digital - A simulator for digital circuits.
BOOLR - Open-source digital logic simulator
Wired Logic - Wired Logic is a pixel based logic simulator.
Logicly - - Design circuits quickly and easily with a modern and intuitive user interface with drag-and-drop...
CircuitLab - Sketch, simulate, and share your circuits, entirely in your browser -- no install required.