There are literally hundreds of competitors out there. But first you have to describe what you like about it. 10-15 years ago everyone would have used something like Request Tracker (RT). There is Redmine, Pivotal Tracker, Trac, Bugzilla. Those are just a few that I recall using. I've probably used more than that.

https://en.wikipedia.org/wiki/Comparison_of_issue-tracking_s...