LogicSim is a logic simulation written in Java. After drawing your digital logic it can be simulated with LogicSim. There are different Modules like FlipFlops ready to use and you can add your own. I have enhanced LogicSim Version 2.4 to support the DeviceServer with 2 new gate types, DevSrv input und DevSrv output. Both Gates have 8 in- and outputs. In the properties dialog of the gates you need to set the URL of the DeviceServer. Due to the fact that the original author has changed the Flipflops in this revision the cube example does not work.
I mailed my changes to the author of LogicSim, Andreas Tetzl. Until my modifications are merge in his code I provide my modified version in the download section under 3rd Party Tools. You need Version OpenMSR V 0.2 . Homepage of LogicSim
The image shows the simulation of a cube, see the Download section.