horde! Actually, horde isn't a webmail system, and it is a web application framework, which happens to have a great mail client written for it, called imp. Imp is a pop3 and imap client. Sendmail, courier imapd, apache, mysql, php4.0, mod_ssl, horde, imp, turba(the address book component for horde), poppassd_ceti(lets users change passwords remotley), and the passwd component for horde together provide everything you need for a really nice webmail system fully supports localization and seems fairly secure.

If anyone is interested in a system like this and doesnt want to set it up themselves, im available for consulting work.

And now, some tips on setting up horde:
before you install horde setup your mySQL server. Each horde app comes with a script in it's scripts dir for making the neccesary tables in the DB.
Make sure to build apache with the newest PHP, and make sure to include support for gettext and zlib.
Untar horde in the apache doc root, then untar each app in the horde dir, that way you dont have to go through horde and change all the paths.
The easiest way to have it do the auth is to tell imp to do it's auth through imap, horde to do all it's auth through imp.
To let users change there passwords install poppassd_ceti, it's like normal poppassd(a daemon for remote password changing), but with PAM support, and install the passwd module for horde. passwd isn't on the main horde page, so you just have to search for it.
When you tell imp what imap server to auth to, the example shows the protocol as "imap/notls" the notls part is not needed, infact, it's an error and will prevent it from working right(it took me over an hour to track this one down).
It works best to just have PHP connect to mySQL via a unix socket, not a tcp socket, but by default PHP tries to connect to /tmp/mysql.sock, and mySQL by default uses /var/lib/mysql/mysql.sock by default, to make the two connect you just have to edit /etc/php.init, there is a line that is something like mySQL defaul_sock, just set it equal to /var/lib/mysql/mysql.sock
in the example config file for kronolith it doesn't mention that there is a mySQL driver for storing calendar data, there is, and it is mentioned further down in the file.
Make sure you have plenty of available disk space in /var
I've found it uasualy works best to built everything static into apache, you will need at least mod_php and mod_ssl, and i suggest mod_throttle(it does http rate limmiting) and mod_gzip, mod_perl(can be usefull for quick hacks), mod_include, mod_log_agent, mod_log_referer, and proably a few others thrown in for good measure. ANd build it all with apache toolbox, http://www.apachetoolbox.com