Web-Based

AnyEMail
Perl program to read/send email from a POP3 email account. [Commercial]
http://www.netbula.com/anyemail/

Atmail
A perl-based feature-rich web email system supporting access via web, WAP, POP3 or IMAP. Fully customizable, with web-admin, scheduler, migration scripts and Unix/Windows technical support. [Commercial]
http://atmail.com/

Auto-EmailPro
Simplifies the process of offering free Web-based e-mail. For mod_perl. [Commercial]
http://buildacommunity.com/autoemail/

Caudium Mail (CAMAS)
Webmail system based on IMHO, designed to run with Caudium Webservers. Designed with the ISP in mind, it's scalable and free to use. [GPL]
http://caudium.net/camas/

EMU Mail
Programs to provide a web interface for accessing a POP3 and IMAP email account. Free version includes banner ads, paid versions don't. [Commercial]
http://emumail.com/

iNetMail
A web-based software module for online mail management. It helps you check and organize your Internet mail through your Web Browser, from an IIS server. [Commercial]
http://www.inetmailonline.com/

NetWin WebMail
Web interface software for accessing POP3 and IMAP email accounts. Available for a range of platforms. [Commercial]
http://www.netwinsite.com/webmail/

Open WebMail
A perl-based webmail system designed to manage very large mail folder files in a memory efficient way. [GPL]
http://openwebmail.org/

Twiggi
A customized version of Twig with additional features, designed to behave like Outlook. [GPL]
http://www.twiggi.org/

WebMail
Provides access to LAN-based email over the Web with most browsers, from a Windows-based server. [Commercial]
http://www.ssimail.com/WebMail.htm

Webmail Pro
Webmail Pro is a robust, reliable, and feature-packed web-based email client driven by ColdFusion. Webmail Pro offers a whole suite of personalized, database-driven features. [Commercial]
http://www.mollerus.net/development/webmail/

WebmailASP
Multiuser webmail ASP script works like a desktop email client. Includes dynamic tree of folders, attachments, HTML-message display, contact books and search. All information moved/copied from POP3 and stored in database. [Commercial]
http://www.webmailasp.net/

WING
Web IMAP NNTP Gateway, an open source Apache/mod_perl based system which allows users to access email held on an IMAP server via any web browser. [Free]
http://web-imap.sourceforge.net/

XandMail
Developers of messaging applications technology for companies and organizations.
http://www.xandmail.com/