RoundCube. Roundcube is a feature rich webmail interface with full address book funtionality, calanders, themes, desktop like interface, drag and drop message management and much more.
Rainloop. Rainloop is a lighter weight webmail application with man less features than Roundcube but is a bit faster. If you generally use a dedicated email client like Thunderbird or Outlook and webmail is something you only use when you're not near your personal mail client, Rainloop might work well for you.
Feel free to try both and choose your favorite. You can always get to both from this page but for quicker access you may wish to bookmark whichever you decide is your favorite.