Raindrop is an open source project by Mozilla. What is should do according to this video is organize your email for you, no need for filtering any more. It should recognize your personal email from the bulk such as emails from Amazon, mailing lists and other less interesting emails. It should also recognize the difference between twitter updates, twitter replies and direct messages from twitter. The promise is also that it will be extend-able so that you can decide which functionality is needed.
We intend to support front-end applications of various kinds (including mobile), but our flagship applications will be built entirely for any modern web browser that supports Open Web technologies. Version 0.1 of our prototype embeds Bespin to support a fast, iterative development style.
For a more technical explanation: