At Giawerx http://giawerx.com we use Mantis http://www.mantisbt.org/. We use the hook scripts in Subversion to have our commits insert comments directly into our Mantis Issue Tracker. Additionally, our website is developed in Joomla http://www.joomla.org/. By forcing Joomla to use LDAP authentication, Our Joomla users are created in our LDAP database, which our subversion server can authenticate against. There is a Mantis component for Joomla, so that autentication is "embedded". Essentially, we have a sinlge sign on implementation for Joomla-Mantis-Subversion. We can add access for employees/subcontractors for various areas of the Subversion repository and the Mantis Issue Tracker. So far, it's been working very well.
We have done some Bugzilla consultation for one of our clients, but they had a very large (50+ developers) developing mostly in Java, C, and Perl, and doing nightly builds. Bugzilla is fairly complex compared to Mantis, and since we're very tied into Joomla for our web content, we tend towards Joomla friendly solutions.
Nancy Hollenback demonstrated the "Unfuddle" subversion/bugtracking hosting at a focused usergroup at my house last week. It's pretty cool, but does cost money. Our implementation utilizes all free software, so we can have as many repositories as we want (i.e Giawerx, PDAwerx, ITwerx, Tradewerx, etc.). We've considered offering a similar service, but for now, we have only implmented it for ourselves.
