We may be eagerly awaiting Mozilla's previously mentioned Raindrop, but webapp Inbox2—already available in beta stage—looks to serve the same purpose: Pulling all your email and social networking communication in a single unified stream.
You can use Inbox2 as either a standalone webapp or a Facebook application, allowing you to choose the hub of your message stream. If you already spend tons of time on Facebook, for example, you can add your email, Twitter, and other updates to your Facebook notifications instead, as well as read and send messages from any of your non-Facebook accounts without leaving Facebook's site.
Inbox2 also integrates well with your existing system, allowng you to import your existing labels and stars from other services like Gmail. It also has support for Microsoft Exchange 2007, and is planning on releasing an iPhone app in the near future (followed by apps for other smartphones). Take a look at their demo video above for a closer look at how it all works.
Inbox2 is a free, web-based service.