A friend has asked me for a way to read email newsletters from within his feed reader, and after some digging around I found it's straightforward enough to access POP3 mail from within Ruby, so I created mailfeed.rb.
mailfeed.rb is a Ruby script that generates an RSS 2.0 feed from the content of an email inbox. This can e.g. be used to read email-based newsletters from within your feed reader. (See also: MailFeed for PHP, pymailfeed for Python.)
It's really a pretty simple script, so there are a number of caveats -- e.g. there is no limit on the number of feed entries displayed, and there is no local caching of mail content (the script will always download the full content of your inbox, which can take a while if you receive large attachments or images).
Also be aware that feeds as communication channels are usually not as private as email. This is especially true if you use a web-based feed reader like Bloglines or SearchFox: your feed might show up publicly in their search results or directories. So I recommend that you either only use this script on a dedicated mail account that doesn't receive private mails, or that you at least configure your web server to require authentication to access this script, and read the feed with a locally installed feed reader such as NetNewsWire or FeedDemon.
The script's comments have installation instructions and more detailed information.
Any feedback is welcome. Tell me if it is or isn't working for you, or if you have ideas for improvement.