Correo is an open source mail app that aims to blend Camino with Thunderbird to create the best darn OS X email client out there. This 0.1 isn't exactly feature rich, but you can't expect lots of features from a 0.1 release. At the moment you can check both IMAP and POP email accounts, send email via SMTP, and choose a 2 pane view or a 3 pane view. Here's hoping that Correo really takes off, since OS X is really lacking a top notch email client.
Thanks to everyone who sent this in.