We are using Bugzilla. Team of 5 developers and 4 > testers. In my previous company we also used Bugzilla with 20 developers, many more testers and project owners (where I inhereted that program to my current job). Although I/we have no knowledge of Linux/Perl, we can pretty easily use it. Bugzilla has a lot of nice features.

But we need a nice open source .Net tracker where we can modify.