I have been working on adopting an old PLC software to support Propeller.
The concept is straight-forward: You draw the LADDER logic diagram in a GUI, switch to simulation mode and simulate your ladder program. When the simulation is done and you ladder works as expected, simply hit F12 and download the prop code into your demoboard!
As you can set all the IO assignments for your ladder diagram, you can develop your own PLC (or use Parallax relay card, etc.) !
The source .spin file is generated, so you can visualize your ladder variables with TVtext, implement special functions (like I2C expanders, etc.)
The software is totally free, and I am still searching for ways of improving this tool. (You can download the software from my homepage - please select English language
If you are an Assembler programmer, you could be interested in FlowProg as well.
Ideas, comments are welcome.