i.Scribe

i.Scribe supports plugins (maker MemeCode Software offers a handful, including spell-checking, on its web site), lets you preview mail on the server before downloading, and runs without modification from removable drives.

A $20 commercial version, called InScribe, adds support for unlimited send and receive accounts (i.Scribe is limited to just one), expanded message filtering, and the ability to synchronize your mailbox with Outlook's.

--Scott Spanbauer