Collaborator is flexible enough to integrate with any external issue-tracking system: JIRA, TeamTrack, Bugzilla, FogBugz and so forth. Integration can be done in several ways depending on the type of integration you require.
It is common to want to associate one or more issues with each review. The easiest way to do this is to create a custom review field and set the regular expression validator to ensure that only properly-formatted issues are entered in.
You should also set up the issue-hyperlink feature described above so that these fields are interactive.
Sometimes you find a defect during review that you do not want to fix just now. In this case you want to move the defect from Collaborator into an external issue tracker. You also want to record this state and audit trail in the review.
The externalized defects feature allows you to do just that. See that section of the manual for details.
This is the extent to which most of our customers go with integration. Most people do not want review defects mirrored into an external issue tracker because these defects were never "delivered".
For example, if you had done the peer review side-by-side with someone, you would not enter in defects then -- you would just fix them!
Usually the QA department runs off the issue tracker for verification. How can QA verify something that was not necessarily externally broken (for example, some function was not checking input values carefully) or something just in the file (for example, some method was not documented properly).
You can use the server-side trigger that runs every time a defect is added, edited, marked fixed, or deleted. Your trigger would locate the corresponding external issue record and update it. This integration takes a bit of work. We do not have it pre-built because everyone's external issue-tracking system is different.
For an example see this scripting tutorial.