Raindrop is an exploration in messaging innovation being led by the team responsible for Thunderbird, to explore new ways to use Open Web technologies to create useful, compelling messaging experiences.

A central principle behind Raindrop is that messaging should be personal — they want Raindrop to be people-centric both in how they process messages, and in how they can help give people control over their personal data and experiences.

When a friend’s link from YouTube or flickr arrives, your messaging client should be able to show the video or photos near or as part of the message, rather than rudely kicking you over to a separate browser tab.

Notifications from computers and mailing lists should be organized for you, not clutter your Inbox or require tedious manual filter setup. It should be easy to smoothly integrate new web services into your conversation viewer entirely using open web technologies.

Source: http://mozillalabs.com/raindrop/