Surprised to not see Moz on here: while I think they were technically a consultancy at first, their chief export for quite a few years came in the form of incredibly valuable blog posts. My favorite is the 91-point eCommerce conversion checklist:
I sort of put 37signals in that bucket as well even though they did a lot more consulting, their blog was pretty huge in the very small standards web community of the early Zeldman era.