Circuit Tree - Design and order Boards in Minutes
Logisim - Logisim is an educational tool for designing and simulating digital logic circuits.
CircuitLab - Sketch, simulate, and share your circuits, entirely in your browser -- no install required.
Logicly - - Design circuits quickly and easily with a modern and intuitive user interface with drag-and-drop...
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.
Digital - A simulator for digital circuits.