Blogs, blogs, blogs
I have 3 clients, and 2 of them want blogs. I have a girlfriend that is lucky to have a tech-savvy boyfriend (depending on who you ask), and she wanted a blog. I even want another blog (for reasons that I will discuss later).
All web developers would benefit from knowing how to design around the constraints of WordPress. So go get a copy of the source, and start playing around.
Web marketers, get to know services like Feedburner, the Tag Generator, and Technorati.