Mac: Inbox by Gmail is a bit polarising, but if you count yourself amongst its fans, then you might want to check out Boxy. Boxy is a Mac client for Inbox that ports the web app over as a desktop app.
Boxy looks and works a lot like Inbox already does. Your emails are divided up between your Inbox, Snoozed, and Done. You can pin important emails, and the interface should be pretty familiar to anyone who's used Inbox. As you'd expect, it also supports multiple accounts, works with any Gmail account, features native OS X notifications, and supports keyboard shortcuts. Boxy isn't really a revolutionary new take on Inbox, but it does bring the web app to desktop in an elegant way. If you don't care about things like notifications or the sleek interface, don't forget that you can always make your own apps from web apps with Fluid.
Boxy ($5.99) [Mac App Store]