Pixel Currency
Beyond Blood
Pro-Folio
Pixelate
Frameworks
Looper
Slim-ent
Graphic designers may find more use for this plugin than web designers, but it can save a lot of time looping through repetitive scaling/duplication tasks.
Sketch Hunt
This is a really interesting plugin because it can create some wild visual effects and patterns in a way that you might not usually consider Sketch to be good at.