There are various options:
Horde/IMP mail client : Its quite a feature rich client, but also quite complex to set up. It gives you quite some extra features if you ready to install the entire Horde suite.
Roundcube: An excellent webmail system. It's quite lightweight, and is easily installed. If you just want a webmail. this should work out for you.
AtMail: Free/Open Source version of AtMail webmail system. Easy to set up, but it only supports POP3, no IMAP.
Regards
Amitabh