Coupon Code: jupb1210 (Checkout as normal and apply coupon before finalising order).
PostBox is a well known email client that was originally forked from Thunderbird. It has a clean UI and lots of additional features on top of what Thunderbird already offered; most important to me - threaded conversations (Gmail-style, which Thunderbird got through an addon/extension albeit not as clean).
Pricing: PostBox used to cost $19.95. But since Mozilla announced that they were no longer going to work on Thunderbird, PostBox dropped their single-user license to $9.95. Using the coupon code (which is provided for sharing Postbox on Facebook/Twitter), the cost of a single-user license is $4.95.
Features and Add-ons: Full-Feature List: http://postbox-inc.com/features
PostBox and Thunderbird do share code but the addons are not the same. PostBox addons can be found here: http://www.postbox-inc.com/extensions/
My Take: I've always liked PostBox for the far cleaner UI, better Gmail support (Archiving, Labels, Gmail Keyboard Shortcuts) and integration with LinkedIn and Dropbox but couldn't justify paying $19.95 but I can't resist it at $4.95. I'm expecting negative votes since Thunderbird's free price is better than $4.95 but PostBox delivers a better application in my opinion. Hope it's useful to some of you.
Great value, I've been using it for months.