Having somewhat recently migrated to OS X, I’ve decided to move from Thunderbird to OS X’s built in Mail.app for it’s OS integration features.
One nice thing about Thunderbird is it’s plugin architecture. Here’s a seriously exhaustive list of plugins for Apple’s Mail.app that might even surpass that of Thunderbird: Apple Mail Plug-ins and Tools.
The one’s I found most useful: Letterbox and MailTags. Mail away!