I have not seen any Open-Source Filemaker Plug-Ins (excepting one written in LISP (http://weitz.de/regex-plugin/) ... Spolsky would have something to say about that). There are some templates available (24uSoftware) and of course those distributed on the Filemaker Advanced media.
That said, I would urge you to become a member of the fmplug-prog-list at Yahoo Groups (FileMaker Plug-in programmers list). There are other plug-in programmers there who are not hesitant to lend a helping hand and advice.
In addition, you might try looking around at http://filemakerplugindev.com . There are interviews with plug-in developers, tips and suggestions regarding languages and IDE's.
regards...
Bob Minteer