Some FileMaker plugin publishers with a proven track record include:
Years ago I did two presentations at FileMakerDevCon, one on plugins, and the other about credit cards. Thus, this page. I’ve updated it a few times over the years, but probably won’t continue. Since then I’ve used plugins happily when required, but in general I’ve come to rely on them less as my needs changed and as FileMaker expanded its native functionality. These days the only plugin I’m using is Dracoventions Developer Assistant, which is super useful to me but isn’t deployed to our users. If I was doing most of my work on Mac I’d probably be using the MBS plugin as well, but since it doesn’t do all of its cool stuff on Windows, I don’t.
You can also find plug-ins at claris.com.