Dmitri Popov has written a useful little tutorial 
on “using Thunderbird like a Pro” at NewsForge.
If you are into that sort of thing, you will find there a quick explanation of how to make smarter use of smart folders, an introduction to some useful extensions and details on setting up turning Thunderbird into a “collaboration tool” with the Calendar and SynKolab extensions.
The last time I checked, that SynKolab extension would only work on Windows. :(