I know you've read from me time and again that I am a big proponent of Google's Gmail interface. However, ever since Mozilla put out Thunderbird 3.0, i've been trying it. It combines the best of both worlds, offline (even though Gmail just released that non-lab), client access, OSX integration. But perhaps the best thing is that they have an archiving system now.
You read a message and you mash "a" and the message is placed into an archive by year-month timestamp, and is no longer in your inbox. The simplest way, client side, to maintain Inbox-Zero.
Take a look at all the new features here.
Please leave comments below.