Raindrop is a Mozilla Labs project that could simplify the e-mail experience by increasing your inbox's signal-to-noise ratio, along with the contextual information about a message. Raindrop's mission is to "make it enjoyable to participate in conversations from people you care about, whether the conversations are in email, on twitter, a friend's blog or as part of a social networking site."
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Though the Thunderbird team at Mozilla is developing Raindrop, the group explains that Raindrop is not another e-mail client, but rather, a communication application. I don't completely agree with this distinction. Today's e-mail clients are communication applications, aren't they? They may not be as well suited to handle current Web communications as Raindrop can, but they're still communications applications. In any case, I'm pretty excited about Raindrop and suggest you watch this demo video.
Raindrop can intelligently categorize e-mail based on pre-built and user-defined extensions. For instance, Raindrop's design lead Bryan Clark asks: "why should advertisements from an Airline push an email form my mom further down the list. I know one is more important than the other, why doesn't my email?"
Another example could be grouping e-mails from Twitter based on their conversational importance. Notifications of direct messages and replies are more important to a user than notification of a new follower.
Message categorization is just the beginning. Raindrop aims to support any message source on the Internet.