Seth Godin is the smartest marketing guy out there. Before I started reading this I had products that failed, but I didn't know why. Now I do. More than any other blog I read, I find myself forward his posts to other people. He doesn't pretend to know the answers, but he asks a lot of great questions. Mostly he takes news stories or things that have happened to him recently--everyday mundane things--and shows you a brilliant marketing lesson you should apply to your own work. I also recommend his books THE DIP and TRIBES.