Catching up on some emails, so here's an announcement that Postbox went live with their beta last week. Some initial good reviews from CNET and Lifehacker. It seems their initial beta has only a Mac version, but I know a Windows version is planned for. More from their site:
A new kind of messaging application, the Postbox™ email client helps you spend less time managing email and more time using email to get things done. Postbox automatically analyzes your e-mail messages, documents, photos, and links to web pages; then it catalogs all this information making it faster to search for and retrieve.
Easy-to-use tagging features let you organize messages the way you want and then focus on one project at a time without missing out on important new messages. You can also gather messages by conversation, annotate messages and create to-do items that appear in mailbox windows.