Artboard manager - Pretty basic plugin, but helpful to keep things organized. Not perfect since it sometimes gets in the way and you cannot disable it per artboard/page.
Potrace - Turns bitmaps into vectors. Doesn't provide a lot of customization, but the results are generally solid. I use Sketch for my illustration, so I don't have another vector tool.
Ohh and I forgot about this little one - https://github.com/gaddafirusli/Preview-in-browser - super helpful for web designers ;)
A couple of my favorites:
Quick note, looks like Artboard manager added the ability to exclude individual artboards by prepending characters to the artboard name.
I'm a big fan of Zeplin, so I'd say the zeplin plugins is a must have if you want to share your work with a front-end developer