What are some alternatives?
When comparing Flyspray and Mantis Bug Tracker, 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.
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.
Bugzilla - Bugzilla is a "Defect Tracking System" or "Bug-Tracking System".
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.