Fossil's bug tracker only works with the web interface or the command-line interface. There's no native GUI client supporting it. There are some independent GUI clients out there, but none of them support Fossil's full range of abilities.
Taiga tries offering a complete Agile software development toolset. It includes complete solutions for issue tracking, videoconferencing, documentation (in the form of a wiki) and either a backlog or a Kanban board for managing user stories.
You can use BitBucket for free if you have 5 or less users (or try it out for free). Price brackets are for 10, 25, 50, 100, or unlimited users (it costs $1 per user per month, or $200 for unlimited users).
Most of the 3rd party tools ( bug trackers, CI servers, chat servers, etc) always integrate with GitHub first, and either don't integrate with Bitbucket at all or do sub par job in it. For example there are 3 separate projects to allow building and verifying pull request submissions on Jenkins...