A great list of mailing list plugins for WordPress (link is offline as of 7/2011). I also found MeeNews at http://www.wp-newsletter.com/. I have not tried it but it looks fully featured.
But I suspect it will be hard for any of these plugins to keep up with all the features that a paid service like MailChimp offers. I recommend the simple plugin the MailChimp team makes. It makes the job of adding a sidebar newsletter signup form a breeze. Note that I often tweak the CSS to get the “subscribe” button to align right or match my theme in other ways.
In this article he needed the ability to export the user list in .csv format so it can be used in other programs (like CRM). UPDATE: When I had a client recently who used MailPress (which i can’t recommend) I was able to extract the email list from the WordPress database using phpMyAdmin and Excel but such a procedure is not for the faint of heart.
via A Trek Through WordPress Mailing List Plugins – Email Mailing List Manager – Social Media Marketing (link is offline as of 7/2011)
See also this great list of newsletter plugins.