My favorite features in new Thunderbird are: the Tabs, Easy Configuration Wizard, the Attach reminder (I forgot it every time) and Skinning
Download the new Mozilla Thunderbird 3 here
Relese Notes:

• Tab Funktionality
• New Search with filtering and timeline tools
• Better Message Archive
• Customizing via Skins
• Smart Folders for multiple Email Accounts
• Addons-Manager (Firefox like)
• Easier Configuration with Mail Account Setup Wizard [Its sooooo easy =) ]
  
• One-Click Address Book
• Attach Reminder, to not forget the attachment
• Activity Manager
• Updated Junk and Spam Filter
• Automated Update System (Firefox like)