Roundcube Webmail is a browser-based multilingual IMAP client with an
application-like user interface. It provides full functionality
expected from an e-mail client, including MIME support, address book,
folder manipulation and message filters.
HTML5_Notifier is a Roundcube plugin.
It displays Desktop Notifications like the ones you might know from Google Mail. Just keep Roundcube opened in a (minimized) tab and enjoy getting notifications every time a new mail arrives.
It just works in modern browsers like Google Chrome, SRWare Iron or Firefox, because "Desktop Notification" is a new feature in HTML5.