What is the simplest, open-source webmail frontend available?
http://roundcube.net/ looks pretty good and I've heard good things about it.
Personally, haven't used it in any huge capacity.
The other commonly used one is horde/imp which is used in a lot of various places, relatively simple from an end users point of view not so much so from a sysadmin point of view.
Hope that helps.
Squirrelmail is probably the simplest, but that doesn't necessarily equate to ease of use. I does support IMAP though, and customizing should be do-able, it is built in php.
http://squirrelmail.org/index.php