SquirrelMail is a webmail program written in PHP4. It includes built-in PHP support for the IMAP and SMTP protocols, and renders all pages in pure HTML 4.0 (with no Javascript) for maximum compatibility across browsers. It has very few requirements and is very easy to configure and install. SquirrelMail includes many features, including address books, spell checker, MIME support, and folders.

In addition to its basic features, various plug-in packages can be used with it, expanding its functionality. Some of these plug-ins include address book importation/exportation, calendar, IMAP password changing, and weather display.

Squirrelmail is open source and is released under the GPL. More info can be found at www.squirrelmail.org or by searching for 'Squirrelmail' on SourgeForge.