Here's my first plugin, which lets Indigo send and receive messages with the OS X Messages app. I wrote it because I wanted to make turning the water on and off at our vacation house super simple for my non-techie family members. And I wanted to learn Python and it seemed like a good little project.
The basic idea is that you set up a device to send and receive messages. Received messages can be retrieved from the device state, and there are plugin actions to mark a message as read and to send outgoing messages.
For both instructions and the download, see: https://github.com/gazally/indigo-messages
The plugin relies on an Applescript handler to collect messages and pass them onto Indigo so installation is a tad more complicated than usual, but I've put instructions in the github README for how to do it. It seems likely that Applescript handler support is not very high priority for the Messages development team at Apple because the El Capitan version, the only one I've tried it on, has a couple of bugs. I don't have a solution for either one of them, but they're not hard to work around either. See the end of the README for details.