We love Middleman for static sites; curious why you chose to go with a server instead of S3+Cloudfront or Netlify. One of the biggest reasons we use static site generators is the ability to CDN the whole thing, making it dirt cheap and blazing fast.
In addition to https://www.oppslist.com (as some have already mentioned), check out https://www.indiehackers.com. Although not all the businesses are entirely passive per se, there are a lot of interesting stories and projects that you might find inspiring.