There are a couple of useful plugins when running a static website exported with Simply Static.
Autoptimize is a free plugin that optimizes your assets (CSS/JS) and can also optimize your HTML output. It’s pretty easy to set up and provides good results in optimizing the rendered output of your website.
It can greatly improve the speed of your static site as well.
Contact Form 7
When using a form on your static website I would recommend using Contact Form 7. It is easy, free, and pretty flexible. Also, it’s directly integrated with Simply Static to use on a static website.
This list will grow in the future so make sure to check out new recommendations regulary.