One of the better solutions is to use a www based mail client, these can usually be run under a secure www server with minimal extra work, and have the added bonus of letting users check email safely from locations that would normally make checking email difficult (while on vacation in Europe, for example). Unfortunately the majority of www-based mail reading clients stink, and the good ones cost an arm and a leg.
AtDot is GNU licensed and written in Perl. It has several modes of operation making it suitable for a variety of www based email solutions (hotmail style providers, ISP's, etc.). You can download it from: http://www.nodomainname.net/software/atdot/.
IMP requires the Horde module (available on the same site) and a www server capable of PHP3 support. You can download IMP and Horde from: http://www.horde.org/imp/.
Written by Kurt Seifried