Postbox is an email client for OS X and Windows built on Mozilla Thunderbird code. But Postbox is much richer in features compared to Thunderbird, including a number of social features and connections to web services like Twitter, FriendFeed, and Facebook. Postbox indexes your messages, contacts, images, links, and attachments, making it simple to search for items stored in your emails. It comes in two editions, Postbox Express which is free, and the full featured client for $9.95.
Magellan Pro is a very flexible client, powered by an embedded database for better performance. It has excellent multilingual support (Unicode, MultiScript, Bidirectional support), provides auto processing of messages, a built-in spam filter. It uses "Views" instead of folders, like Opera M2 and GMail.
Entourage is Microsofts email client for Mac OSX. It can connect to POP, IMAP and Exchange servers and also provides native support for Hotmail. Even though a bit slow, this is a powerful email client.