<p>Does anyone know of a good (preferably open source) library for dealing with the Modbus protocol? I have seen a few libraries, but I am looking for some people's personal experiences, not just the top ten Google hits. I figure there has to be at least one other person who deals with PLCs and automation hardware like I do out there.</p>

<p>Open to any other materials that might have been a help to you as well...</p>

I have done a lot of communication with devices for the past few years, since I work for a home automation company, but we don't use Modbus.  We do communication in a standard and open way using Web Services for Devices(WSD) which is also know as
Devices Profile for Web Services(DPWS)
.
During this time at one point, I did hear of a project called
NModbus
.  It is an open source library for working with modbus.  I have not used it, but looking at the site and the
changesets
on Google Code, it looks pretty active.  You may want to give it a look and even get involved in.  This is the only library that I have heard of that targets .Net.