Eventum

Eventum is a user-friendly and flexible issue tracking system that can be used by a support department to track incoming technical support requests, or by a software development team to quickly organize tasks and bugs. Eventum is used by the MySQL AB Technical Support team, and has allowed them to dramatically improve our response times.

Eventum is a pretty complete issue tracking program. That said, it does require some additional work if you want to have customers available in Eventum. There is an API that allows you to integrate with your existing customer database if you have one. Alternatively, I would be nice to see a solution that uses
something like SugarCRM if one doesn't have have a customer solution.

The user interface could also use some improvement. Putting the entry screen above the list of records is cumbersome, and at a minimum should have a DIV to hide/show it.

However, it is probably one of the more professional open source bug/issue/ticket tracking systems available.