Windows/Mac/Linux: Mozilla's email and messaging application is moving toward a 3.1 release for June, and little helpful things are popping up in the betas. Among the latest are a new "Quick Filter" bar and better Thunderbird 2 upgrade/import tools.
Users making the trip from Thunderbird 2 to 3 had a right to feel badly looked-after in the initial release, as the import tool didn't do such a hot job of keeping everything in order. That's improved in this second 3.1 beta, as have a good number of other problems and bugs. The other notable new feature is a Quick Filter bar that quickly shows messages from contacts, that you've starred, unread, have attachments, or other common filters inbox cleaners like to have.
Better font defaults, auto-complete, tab behaviour and other fixes are detailed in the release notes. Thunderbird 3.1 beta 2 is a free download for Windows, Mac and Linux systems.