IMAP is now sufficiently popular that finding IMAP clients is no longer difficult - most popular mail clients have IMAP support, or will do soon. The following screenshots show 3 IMAP clients in use - Netscape, Mutt, and IMP. All three are using different forms of encrypted authentication, and are viewing the same mailbox.
Several IMAP webmail clients exist - these can vastly simplify life for roaming users, who can then access their mail from anywhere with an internet connection. Examples of these are SquirrelMail (http://www.squirrelmail.org) and IMP (http://www.horde.org/imp/2.2). Both of these require additional configuration work, as they are PHP based. Squirrel Mail uses its own implementation of the IMAP protocol, and, for this reason is probably easier to set up than IMP, which requires an external library distributed with the UW IMAP server.