Engineers blogging about Solaris etc. Internals, gotchas, upcoming things, heads-ups on problems. The patching guys' blog was particularly useful in the face of Solaris' atrocious packaging utils, for example.

There was a _huge_ amount of official-unofficial documentation in the blogs that disappeared from the Sun site when Oracle ran its bulldozer through it.

Sun very strongly encouraged their employees to have blogs, and was either one of or the first to do so (especially at their level). Basically no restrictions on subject matter. Even their CEO regularly blogged!

A better question would be "What didn't Oracle do to kill the Sun blogging culture?"