Zimbra
Anyone who has suffered the abject misery of using Microsoft’s Outlook Web Access will be interesting in this new, Open Source offering - Zimbra. Zimbra uses AJAX among other things to provide a rich groupware application (email, calendar, tasks and so on) direct to your browser. The server side component also supports the open standards IMAP, POP3 and iCAL as well as Microsoft’s MAPI (whatever).
A full demo is on their website.