Next.js is technically open-source, but unfortunately a lot of more powerful features depend on Vercel (who owns it)... it's actually bad enough that I'm worried about vendor lock-in, and wish I had done more due diligence upfront. It's only getting worse too, with features like Next.js Live collaboration (think GDocs for websites) being built around Vercel infrastructure.
Gatsby is another competitor, though way too complicated (requiring a transformation step to GraphQL, I think) and pricy/slow. Github Pages for similar sites.
For the CMS part of a jamstack, I really like DatoCMS for its editor-friendly features & GraphCMS for its dev-friendly features. Both are better IMO than the industry leader, Contentful (which is insanely overpriced).
Outside of serverless hosts, I love Datadog's synthetic tests for simulating & testing user journeys across pages.