Fluent is a new web-based email client that turns your Gmail inbox into a fluid stream. It's styled much like Sparrow, our favourite Mac email client, works with multiple accounts, features fast search and has lots of other features.
Right now, however, Fluent is in closed beta. You can try the demo to see how Fluent's "workflow-oriented stream" works and sign up to be notified when it opens to the public. The service looks very promising, with the ability to turn emails into to-dos in one click, attachments in one place, and quickly go through your email stream.