Windows/Mac: Previously mentioned email client Hive Five contender Postbox just released a free version of their powerful email client. Postbox Express is missing some of Postbox's best features, but isn't without its great Gmail-like searching and organisational powers.
We loved Postbox back when it was in free beta, but after the final version went on sale for the slightly ridiculous price of $US39.95, most people jumped off the bandwagon. Now, fans of the advanced email client can get their fix of some of Postbox's features with Postbox Express, most notably Gmail-like search operators (such as from:jason) and threaded conversation views.
Unfortunately, it's almost a little too toned down. Most of the features that made Postbox great, like file, image and URL search, are completely missing. Even conversation views are limited to messages in that folder, as opposed to finding all messages on the account from a specific thread. It's better than nothing, but it's also not as compelling as we would have expected (it is, however, free). Postbox has a handy comparison chart on their web site that breaks it down further.
The bottom line: if you've already paid money for Postbox, then don't feel bad, because you probably wouldn't be happy with the free version anyway. But if you were a beta tester that didn't find Postbox worth a whopping 40 doubloons, this is probably a pretty good compromise.