We've covered Postbox in the past, and I've been using it regularly since the beta. Version 2.0 was just officially released, and there's a lot to love in the latest incarnation of this Mail.app replacement.
Postbox already had some great organization capabilities, including "Conversation" views for email threads, and "Topics" for tagging messages and quickly sorting them. With version 2.0, organization is faster and even easier, thanks to features like Quick Move and some new keyboard shortcuts. There's even a Quick Reply feature which lets you respond to a message from within the messa