Sup is an attempt to take the UI innovations of web-based email readers (ok, really just GMail) and to combine them with the traditional wholesome goodness of a console-based email client.
Sup is designed to work with massive amounts of email, potentially spread out across different mbox files, IMAP folders, and GMail accounts, and to pull them all together into a single interface.
The goal of Sup is to become the email client of choice for nerds everywhere.
The current version of Sup is 0.0.1, released November 28th, 2006. This is an alpha release. It is unix-centric, mbox-specific, and has no i18n support. I plan to fix all of these problems.
Other than those limitations, it works great. I use it for my everyday email.
You can download Sup releases from the Sup RubyForge page. If you have RubyGems installed, simply say "gem install sup".