What are some alternatives?
When comparing Mantis Bug Tracker and Jira, you can also consider the following products
Redmine - Flexible project management web application
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.
Trac - Trac is a project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
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.
TeamClerk - Team Clerk is project and task management software that automatically keeps your team organized and monitors progress of all of your projects and tasks. Let us organize what you do,so you can focus on why you do it.
Wrike - Wrike is a flexible, scalable, and easy-to-use collaborative work management software that helps high-performance teams organize and accomplish their work. Try it now.