The beta allows you to reply and compose mail. It does not require a POP3 server, or any shell access for the users. It interfaces directly with user mail spools. By setting up W3Mail on your website, your e-mail users can access their POP3 mail from anywhere in the world, as long as they have access to a web browser. It also has MIME support, folder manipulation, etc.
Twig does IMAP and newsgroups.
CWMail by NetWin is for businesses. DOES NOT support traditional Mailbox files, only Maildirs. This is the same webmail server that's included in the Courier mail server, but packaged independently. If you already have Courier installed, you do not need to download this version. Features include address book, filters, signatures, checking external pop accounts, and ICQ notification of new e-mail.