@Kerbonklin: the OP was asking about IGM, not RM, so your answer doesn't apply here.Plug-ins, no.
However the next closest thing are scripts that make use of note-tag / comments put into the database and events that add new functionality that the default engine does not have.
The IG Maker is relatively new, and I don't use it myself. I know that there is some sample data available for it on this site, and I expect more to become available given time - but as fas as I know currently the number of plugins is limited. But please wait a few days until someone from the IGM-community with a better knowledge of IGM can read and answer your questions.Can you download more plug-in's to increase the functionality of this engine?