Best Integrated Issue-Tracker for Subversion?

JobSeeker writes: "Now that subversion has merge tracking my boss wants to save money by dropping our expensive commercial solution. I've pretty much convinced myself that subversion can do the job. I like it. But what about integrated issue tracking? Version control without issue tracking is only half a solution. The TortoiseSVN docs say a little about bugzilla and not much else. What ready-to-play options (commercial or open source) exist for deploying subversion on commercial projects?"