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])