UPDATE: MSDN and TechNet Blogs have been closed down. There are similar blog options now available over at Microsoft Tech Community (https://techcommunity.microsoft.com/).
But this topic is more for the community (people who are not Microsoft employees).
You must be a Microsoft employee to create a blog. (It does not matter if you're part time or a full-time employee.)
However, non-Microsoft employees, contractors/vendors, interns and others can join a team blog that's being run by Microsoft employees. Because most team blogs represent products, those product-based blogs are typically only run by Microsoft employees (occasionally with Contractors added as authors). And then there are many personal blogs that are created by employees for themselves to blog on (non-Microsoft employees and contractors cannot create personal blogs). But a few other blogs exist to represent the community, such as the TechNet Wiki blog or MVP blogs about a specific technology or language/area. Most of these community blogs are so specific that they don't have a clear process to join them. When those processes become transparent (like the Wiki Ninjas Blog), we will add links to those processes below.
These are blogs that include authors who are not Microsoft employees. Note that these blogs are started and owned by Microsoft employees. And the point of the blogs is to highlight community participation (these are not personal blogs and are not blogs that only focus on the official product info). If you find any blogs with community authors that are not yet on this list, please add them!
Also, note that most all these blogs don't have a process for non-MS community members to join. Most are by invite only (which makes sense, because they are very targeted, like MVPs who contribute heavily to TechNet Wiki in the Portuguese language).
The blogs are listed alphabetically:
For Microsoft employees: