Thunderbird 3 Beta 2, a preview release of the upcoming version 3 of the free e-mail program, will look entirely familiar to anyone using Mozilla's excellent Thunderbird 2. It showcases a few new features, such as archiving and a new activity manager, but most of the changes are under the hood.
The new Archive feature (inspired by Gmail) automatically moves a message to an archive folder when you hit "A" with a message open or select Message | Archive. The archives folder layout sorts messages by year, with subfolders for each month.
The other new addition, the Activity Manager, show all interactions between Thunderbird and your e-mail account server(s)--such as sending and receiving mail--in a new pop-up window. Thunderbird 2 doesn't do a good job at telling you what it's doing, and the Activity Manager should make things more clear.
These new additions are nice, but probably won't change much for your day-to-day emailing. And aside from some handy new buttons for functions such as Reply, Forward, and Archive, which show up in the header information bar next to the from: and to: info when you preview or open a message, you probably won't immediately notice most of the other changes. But there are a slew of bug fixes and tweaks in the new version, such as faster message loading for IMAP accounts. See the release notes for the full list of changes in this beta.