The Electronic Freedom Foundation- Legal Guide For Bloggers
Everything the EFF does is good and deserves to be read. This guide covers legal liability issues for bloggers and should be your first read before thinking about blogging.

IBM - IBM Social Computing Guidelines
The content of this guide came from a wiki IBM created for it's employees who were interested in blogging. It's a series of best practices and guidelines. For the most part the IBM guidelines are common sense but I still think the document does a great job compiling and presenting it's ideas.