I’ve been really satisfied with Sparrow for Mac. It’s a new mail client that does email very clean and streamlined. It supports Growl notifications and stays pretty much out of the way until you actually get new mail or until you need it to compose a message. It syncs seamlessly with my Gmail account and just about everything I can do in the browser version I can do from here — that includes tags, labels, archiving, etc.

If you’re sick of keeping a browser window open just for Gmail (without the chat) then I recommend giving Sparrow a go.