The list of companies, organisations and projects using Bugzilla has recently passed 400. (The numbers are always a little vague, as public Bugzillas come and go.) The list includes such major projects as Mozilla , the Linux kernel, GNOME, KDE, and Apache.
Credit goes, of course, to the great team which keeps the whole show on the road. And as that page goes to show, people don’t choose a bug tracking system based on the attractiveness of the developers. ;-)