I have the NG Flash Toolbar, but I only use it to make my theme compatible with it as I dislike the money-making, user-tracking enterprise that conduit toolbars normally represent(mind you, not all conduit toolbar 'creators' take advantage of this).
I also have the web-developer toolbar as it has helped me often and the yahoo toolbar, because it is written in a similar way to the google one and I want my theme to be ready for it.
And finally I have my own personal toolbar that I created and do not intend to release.
I normally only have one toolbar visible at any given time as this saves on space and eliminates any toolbars that are unnecessary at the time leaving an uncluttered, easy to use Firefox 3.