One of my favorite newsletters for marketing inspiration is Springwise. It's not always about Marketing, but is really about taking a different approach to your work. Working in the boundaries can be suffocating – it's nice to think outside them!
You can sign up for the newsletter here. Another nice service they add is the ability to download a pdf of the newsletter for distribution. The site is set up more like a blog and has an ‘idea database'. Check it out! Inspire yourself!