For non-technical: I've always thought Buffer[0] and Hubspot[1] were fantastic resources for SEO, marketing, etc. Hubspot even writes extensively on how to write content for SEO/SEM, which is sort of meta.
For technical: Stripe[2] is a popular answer to this question. I also like Auth0[3], especially b/c they often have content that isn't just meant to market their services (eg this tutorial on k8s[4])
[0]: https://buffer.com/resources/
[1]: https://blog.hubspot.com/
[2]: https://stripe.com/blog/engineering
[4]: https://auth0.com/blog/kubernetes-tutorial-step-by-step-intr...