Fluent is a project being worked on by 3 Australian (Aussie..Aussie..Aussie..oi…oi..oi) ex-Google employees. Gmail content is presented in a web-based work-flow oriented stream rather than your traditional inbox format, which allows you to access items more quickly and efficiently. Ease of browsing attachments and searching is also addressed, as well as various other features. You can also work with more than one mailbox within the same window. While, ironically, it currently only works with their ex-employer’s Gmail, plans are to extend it to other email systems in the future. It’s still in closed beta, but as someone who runs 4 Gmail accounts for different purposes, I’ll be watching this project very closely (and I’m waiting on the list for an invitation).
You can go on the waiting list for a signup on Fluent.