Roundcube (default) is provided at webmail.gridhost.co.uk (and securely at https://webmail.gridhost.co.uk). It is very easy to use and makes use of some nice features. You can set up filtering rules, multiple folders and maintain a contact list/address book all from a fast, easy web-based interface.
Atmail (@mail) is an alternative to Roundcube provided at AtMail - Login. It is also very easy to use, and is completely unbranded.