Mac: BlinkMail is an email app, currently in open beta, that allows you to flip through and organise emails with simple keyboard shortcuts.
BlinkMail is similar to Mailbox in that it's mostly made for triaging email, not composing it. Each of your arrow keys is assigned an action you can customise like Flag, Archive, Delete, Move to Folder, or Reply. Then, you can quickly flick through your inbox using just your arrow keys.
While it's not feature-packed enough to replace something like Sparrow, if you need something to triage email with, BlinkMail does the trick. You can check out the beta for free right now.