Currently limited to Gmail accounts, Mailbox opened up to massive queues of people waiting to get in. It wants to help you get on top of your mail and stay on top by prioritizing what matters right now. So, why are we interested?
Besides being a first rate application regardless of platform, the developers have publicly hinted at an Android version being on their roadmap. It's pretty vague, but when questioned on Twitter they did indeed confirm that "other platforms are coming down the road including Android."