ChimpBlast allows you to compose, edit, and preview messages within WordPress before submitting them to the MailChimp broadcast email service. ChimpBlast was particularly designed for use with RSVPMaker and can be used to send email invitations to events. You can also import content from blog posts.
Download page: http://wordpress.org/extend/plugins/chimpblast/
Plugin home page: http://www.rsvpmaker.com/chimpblast-plugin-for-mailchimp/