Probably worth relinking their excellent blog post "Going Freemium: One Year Later"
http://blog.mailchimp.com/going-freemium-one-year-later/
Their 'reverse freemium' approach is really interesting -- rather than de-emphasising the free and focusing on the [pre]mium side of their biz, they've been slowly expanding their free options and growing rapidly -- in terms of profit -- because of it.