Thunderbird has just taken a step towards the final version 2 release as a second beta was announced today featuring a number of significant improvements. From the release notes:

• Message Tags: Create your own tags for organizing email. Messages can be assigned any number of tags. Tags can be combined with saved searches and mail views to make it easier to organize email.
• Visual Theme:Thunderbird 2’s theme and user interface have been updated to improve usability.
• Session History Navigation: Back and Forward buttons allow navigation through message history.
• Folder Views: Customize the folder pane to show favorite, unread or recent folders.
• Improved Support For Extensions: Extensions can now add custom columns to the message list pane in addition to storing custom message data in the mail database.
• Improved Offline Management: Thunderbird (Linux and Windows) automatically adjusts the offline state based on current network connectivity.
• Improved New Mail Notification Alerts:New mail alerts include information such as the subject, sender and message text.
• Folder Summary Popups: Mouse over a folder with new messages to see a summary of the new messages in that folder.
• Saved Search Folder Performance: Search results for saved search folders are now cached, improving folder loading performance.
• Find As You Type
• Improved Filing Tools: Recent folder menu items for moving and copying messages to recently used folders. Move / Copy again functionality.
• Updates to the extension system: The extension system has been updated to provide enhanced security and to allow for easier localization of extensions.
• New Windows installer: Based on Nullsoft Scriptable Install System, the new Windows installer resolves many long-standing issues.
• MacOS X Universal Binaries