I've had a wrought iron knocker plugin for automatically picking bloom and hammering it into wrought iron, and auto chip plugin which I used for chipping boulders during bloodmoon.
They are very simplistic and require constant supervision, especially when you're lagged. As Kandarim said, from the plugin level you don't have many hooks available, and you can't track communication with the server, so it's hard to make them bulletproof. I'll see later if they still work and post them here.
Was working on a plugin to operate bellows too, and it kinda worked but there were some issues with recognizing smokes and it was too slow for pressing bellows on time.
Also, was working on a bird warning plugin for when crows are too close to fields, but havent finished it, so there's an idea for plugin makers.