One of the new features in Thunderbird 2.0 is that extensions can now add their own columns to the list of messages. I always thought that this sounds good and now I've found the first extension that makes use of this feature. But the extension doesn't just add any old column, it actually adds a very useful column if your mail server is running SpamAssassin.
Spamness adds a column that displays SpamAssassin's spam score in an easily scannable graphical way. 
After you have installed spamness, you can easily scan your spam folder for very spammy and not so spammy mails and you can sort it by spaminess. (Install the Mnenhy extension if you don't want to sort all your folders by spaminess and if you don't want to display the spamness column in each folder).