Sparrow is a Gmail-specific email client for OS X that looks and acts like Twitter for Mac. I love it. So does Matt. 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]