A recent project I'm working on needs to behave as a Modbus slave. Not that Modbus is a complex protocol, but it is always handy if you can save yourself the effort of doing your own implementation. After some trouble I've got FreeModbus library up and running.
While looking around I've also found some cool tools like modpoll that can allow you query a Modbus slave. And while I had some trouble with the Linux binary (libstdc++5 was missing), the Windows binary runs happily under wine too. Another great set of tools are these ones. You can even connect your Excel spreadsheet to values obtained from a remote slave!
It was fun to realize there is even a Modbus for Arduino implementation.