Aethera
Althea
Evolution
GnuMail.app
Ishmail
KMail
Mahogany
ML - A mail program for X and IMAP
Mulberry
Mutt
PMail
Sylpheed
TkRat/Ratatosk
TradeClient
Archimedes, formerly known as XFMail
Postilion, a clone of the NeXT Mail.app
An Index of IMAP Products
Test IMAP server
Pros and Cons of the most common IMAP clients