iOS (jailbroken): Sparrow is a fantastic mail client for iOS, but without push notifications and the ability to become your default client, it falls a little short. Jailbroken iOS users can add both of those features with the free Sparrow+ tweak.
Sparrow+, available in the Cydia app store, essentially bundles previously mentioned Sparrow Push with the ability to make Sparrow your default email client. So, whenever you click on an email link in Safari or send an email from your Contacts app, it'll take you to Sparrow instead of the built-in Mail app (we still can't believe Apple hasn't built this into iOS yet). You'll find this setting in your Settings app, while you'll find the Sparrow Push settings in their regular location in the Sparrow app itself. And, you can customise your notifications in the Notification Center.
Sparrow+ is a free download for jailbroken iOS devices.