Mailer and newsletter-related web-based apps.

CcMail is a free, open, powerful Mailing List Manager that doesnt require any programming knowledge. It was designed to have a user-friendly interface, being in the same time complete and powerful. CcMail offers the basic functionalities of a commercial, expensive script for free.
Dada Mail is a Contemporary Mailing List Manager, enabling you to easily run a verified mailing list that your subscribers can trust, right on your own website. Dada Mail helps you with managing an email mailing list, offering complete support for safe, closed-loop opt-in subscriptions, sending out mass mailings, keeping message archives and allowing you to share your messages in lots of neat ways.
OpenNewsletter is a web-based open source solution for sending email newsletters to a subscriber list. Email delivery options include an HTML version and/or a Text version.
phpList is an open-source newsletter manager. phplist is free to download, install and use, and is easy to integrate with any website. phplist is downloaded more than 10000 times per month.
poMMo is versatile mass mailing software. It can be used to add a mailing list to your Web site or to organize stand alone mailings. Unique Features such as the ability to mail subsets of your subscribers set it apart from alternatives.
Postfix Admin is a web based interface used to manage mailboxes, virtual domains and aliases. It also features support for vacation/out-of-the-office messages.
RainLoop webmail Simple, modern & fast web-based email client. RainLoop webmail was designed with efficient memory use in mind, so it can work fine even on low-end web servers. Regardless of mailbox size and number of mails in it, memory use per active user is always kept at a minimum, even when very large mails are processed.
Roundcube - Free webmail for the masses. Roundcube webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation, message searching and spell checking.
SquirrelMail is a standards-based webmail package written in PHP. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages render in pure HTML 4.0 (with no JavaScript required) for maximum compatibility across browsers. It has very few requirements and is very easy to configure and install. SquirrelMail has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation.
WEBinsta maillist manager is built to provide a centralized mailing list / newsletter system for small to medium website . Its simple yet quite powerfull . It support the standard subscribe and un-subscribe , remote verification , unsubscribe links , attachments and host of other such features . The best of all the total integration is less than 3 lines of code .
WebMail Lite Open-source webmail script for your existing IMAP server. WebMail Lite can be used to access mail on virtually any IMAP enabled mail server. The integrated web administration panel allows you to manage the system settings without manual editing config files.