What are some alternatives?
When comparing Mantis Bug Tracker and TeamClerk, you can also consider the following products
Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.
Redmine - Flexible project management web application
Trac - Trac is a project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Bugzilla - Bugzilla is a "Defect Tracking System" or "Bug-Tracking System".