KBiff supports all major mailbox formats: mbox (Berkely style), maildir, mh, POP3, IMAP4, and NNTP. As of 3.7, it also supports POP3 and IMAP4 with SSL. As of 3.8, it supports CRAM-MD5 and APOP authentication.
KBiff is also part of the KDE project. Among other things, this means that it has support for session managment (it "remembers" the last state it was in before you logged off) and can be docked into the panel. It also is translated into several different languages (like this).
A relatively complete list of KBiff's features can be found here
Since the design of KBiff was so terrible, it had become quite unwieldly to extend. I decided to "start from scratch" in the 1.1 development version. This resulted in the version found in KDE 1.x (in the kdenetwork package) as well as formed the basis for the current version.
KBiff was then ported to KDE 2.0 and 3.0 and development slowed to a crawl. It will likely not be updated very much since I don't have very much time to spend on it (there are too many other more important things to do).
I do accept patches, though, and it was one such patch that finally ported KBiff to KDE 4.
See the Download page for links to source and binary packages.