Yeah, I downloaded and started using it. Pretty cool plug-in, I must say (and it is a plug-in, not an extension, which explains why I couldn't find it when you first mentioned it). Love the tab control and bookmark options, etc. I have been using the Sessions extension to restore windows and tabs upon re-launching Safari and some other neat ones as well.
I haven't tried the bookmark customaization tool, but I wonder if it will let you place a bookmark "alias" folder, so that when you add, edit or delete a bookmark or folder in one place you don't have to find and update it everywhere it appears in your bookmarks. For instance, if you had an awesome gigapixel photo of London, you might want the bookmark to be both in a bookmarks folder for "Cities" under "Travel" and also one for "Gigapixel Photos" bookmarks under "Awesome Images", etc. I have needed that for a LONG TIME. Can't believe no one else has.