SensibleEmail

Email client designed to be reliable, and easy to use. All email is kept in a SQL database. Currently supports POP3 and SMTP. The POP3/SMTP access is all performed in a separate process so that it doesn't cause delays in the UI.
The Server process may be left running and fetching email, so when the UI starts, it always has the latest messages.
It always displays both the HTML and Plain Text versions of a message, there are multiple ways to control which one is onscreen first. (by folder, by sender, by default)
Many features designed to make reading Mailing Lists easier. (Wizard filter creator for instance)
Tags on msgs can be used to control font/color of each individual field (Subject, From, etc...) as well as performing actions when the tag is applied or removed. (Spam/Fishy mail is handled this way.)
Folders may be renamed or moved at any time without breaking filters.