Letters
Brent Simmons has kicked off a project to to write a truly great Mac mail client for designers, developers, and power users. The mailing list is already hopping with people — including yours truly — submitting ideas and arguing over defaults. Consequently, the project is still in search of a president to act as a guiding voice.
I like Mail.app just fine, part of me wants to see some of these great ideas submitted to Apple as feature requests, but like anything, competition is the best motivator.
Plus Letters.app is just the perfect name for an email client.