Sparrow is a Gmail-specific email client for OS X that looks and acts like Twitter for Mac. I love it. And so does Lifehacker. And now it's faster and more powerful.
You can now drag your inbox down to refresh (notice how this is becoming something of a universal gesture?), use the built-in CloudApp functionality (good for sending attachments), display email threads like Gmail (oldest first), and expect to see faster load/send/sync times. If you're a Mac user that hasn't yet tried Sparrow, you should. [Sparrow]