MailDB adds a database back-end to an IMAP server for features such as "virtual" mail folders (msgs appearing in multiple folders), "view" folders (dynamic results of searches), etc. The DB is inherently multi-user, allowing msg reference counting.
CSCMail is an X/Gtk based MUA. It supports the POP3 protocal and can also read mailspool files/directories. It supports multiple accounts, filters, GPG, MIME, Spellchecking, HTML Mail (view only) and a host of other features.
pemail allows you to view POP3 email from the command line, much like the UNIX mail(1) command does. pemail also allows you to send email from the same interface, making it pretty much an all in one mail client for the UNIX command line.