There aren't a lot of Microsoft Outlook competitors out there, but Mozilla's open-source Thunderbird is one of the best. Postbox for Windows and Mac, and built on Thunderbird code the way that Flock is based on Firefox, is a new face on the e-mail field.
Still in beta, Postbox takes desktop e-mail hard toward Web 2.0, with fast links to upload contacts to Facebook and pictures to Picasa. Click on an e-mail, and the preview pane not only shows the text, but extracts all links, images, other attachments, and contacts into a sidebar for easy management. Postbox is also obsessed with tabs, which are coming in Thunderbird 3.0 but not to this degree--at least not from what we've seen. Postbox can also upload to Twitter, FriendFeed, MySpace, del.icio.us, and Google.
When Postbox starts, you can import e-mail, contacts, and other messaging data from Thunderbird, Outlook, Google, and Yahoo. Once you get going, Postbox's tabs can be used to filter out the text of a message and focus on only attachments, images, links, or contacts. Messages themselves can also be opened in new tabs, cutting down on the excessive clutter that Web mail eliminated ages ago.
An excellent remix of a Thunderbird feature is Topics. An expansion of Thunderbird's Labels that automatically searches across folders for messages tagged with the same Topic, Postbox's is undeniably what its progenitor's should have been. Thunderbird's Labels can be configured to behave the same way, but it's not done without extra effort.
Postbox doesn't feature all of Thunderbird's abilities. The biggest one missing is that add-ons have been killed. So while Thunderbird 2.0 can be given calendaring via the Lightning plug-in, and Thunderbird 3.0 will integrate Lightning natively, Postbox only has a To-Do list. Depending on what you're using your e-mail for, this may be a major drawback, or much ado about nothing.
Social-networking junkies who are looking for a desktop solution should take a good look at Postbox, or at least keep an eye on its progress. Support for Flickr is hopefully coming, and I'm interested in seeing how Postbox reacts once Thunderbird 3 finally leaves beta later this year, but Postbox is proof that alternative desktop e-mail clients are far from dead.