Open WebMail
IMP Webmail Client
SquirrelMail - Webmail for Nuts!
IlohaMail is a light weight yet full featured multilingual webmail program that is easy to use and install. It runs on a stock build of PHP, and does not require databases (although database support is available) or the IMAP library (it is powered by a custom IMAP/POP3 library).
RoundCube Webmail webmail is a browser-based multilingual IMAP client with an application-like user interface.
V-webmail (dead?)
Null Webmail is a simple yet powerful webmail CGI written in C. Null Webmail is platform-independent and supports more than 25 different languages. Null Webmail is Free Software, and is available under the terms of the GNU GPL.