Micro Persuasion - Steve Rubel's frequently updated blog focuses on how new technologies are changing marketing and PR. IMHO, if you only have one marketing blog in your RSS feed, this should be it.

Blogspotting - Focused and well-researched blog about blogging from Businessweek.

Seth Godin's Blog - Godin isn't as focused on community media as Steve Rubel but his advice about marketing practices is well worth reading.

BlogWrite for CEOs - Debbie Weil's blog on how to apply blogging for business benefit. She knows her stuff. Has a book coming.

Dan Gillmor's blog - Dan may think more deeply about community media than anyone on the planet.  The former San Jose Mercury News's blog is a mix of political commentary and journalistic activism and always worth reading.

Small Business Trends - This site has profiles of dozens of blogging businesses. 

Blog Relations - UK-based blog focused on applications of PR to the blogosphere. Has a frequently update podcast feed.

InFlightHQ.com - This is a marketing blog maintained by Boeing as a service to business travelers. Interesting use of content marketing. 

RoadWarriorTips.com - This blog was set up by Extended Stay Hotels, a Spartanburg, S.C., hotel chain. It's another example of how to use blogs in brand marketing.

The Intuitive Life Business Blog  - Dave Taylor writes a provocative, candid and often very useful blog about communications, marketing and branding. It's one of three he maintains while also consulting and writing books.