Graphical circuit design and Logic simulation: TkGate

TKGate Home Page
Very cool.

TkGate is a event driven digital circuit simulator with a tcl/tk-based graphical editor. TkGate supports a wide range of primitive circuit elements as well as user-defined modules for hierarchical design. The distribution comes with a number of tutorial and example circuits which can be loaded through the "Help" menu. The example circuits include a simple CPU, programmed to run the Animals game. For more information, check out the documentation. TkGate is free software provided with source code under the Gnu Public License.