Zimbra is an Ajax-based webmail. Zimbra is an Open Source Software; it follows Mozilla Public License (MPL). You must see the demo of Zimbra. It shows some of the great features of Zimbra:

• Drag & drop of messages in folders
• Messages in different folders are showed in the same conversation thread
• You can tag your messages
• Powerful search feature can even search in attachments
• You can save your most used search queries
• And more...