Open Source & Ajaxed Webmail: Roundcube
Roundcube is an open source webmail application with a slick, desktop-like interface.
It is multilingual (already comes with 60+ languages) and offers most of the functionality you can expect from a webmail like:
- full support for MIME and HTML messages
- composing messages with attachments
- find-as-you-type address book integration
- Richtext/HTML message composing
- Forwarding messages with attachments
- searching messages and contacts
- threaded message listing
- spell checking
Messages can be managed with drag'n drops, users can have multiple sender identities and it can work with unlimited users/messages.
Roundcube is built with PHP and requires MySQL, PostgreSQL, SQLite or MSSQL for storing the data.
It has a plugin API for building extensions and adding custom features.