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. 

NOTE: This will not build on a stock x86_64 system.