This is just what I was looking for. Craigslist must have changed their page layout since this was written, because it didn't work for me when I installed it.
IT DOES WORK IF YOU CHANGE THIS IN THE SCRIPT:
end of line 11 should change from ['i'] to ['small'], as follows...
var theCategories = document.getElementsByTagName('blockquote').getElementsByTagName('small');
THANKS for a great and useful script.