Mac: Airmail is a well-designed, fast and fully-featured IMAP email client that works perfectly with Gmail or any standard IMAP account. It integrates all the features you'd expect, and you can even automatically attach files with your Dropbox account.
Airmail supports many Gmail features, from priority inbox to labels to proper archiving. You can even remap certain actions, such as rerouting starred mail to a separate label.
Airmail has a bunch of really interesting features on top of that, too. You can choose between themes to alter your message view. It allows you to create separate notifications for different mail accounts so you can track crucial accounts more easily.
If you've used our favourite Mac mail app Sparrow, you'll find a lot of Airmail familiar. With Google's purchase of Sparrow, development has stopped. Airmail seems poised to take over and add a lot to like in the process.
Currently Airmail is in public beta, which means you need to sign up to get a copy. This process isn't difficult; you need a HockeyApp account and you'll (seemingly) be automatically approved for download. To sign up and get a copy, just follow the link below.