It's quite big in the free software world. Trac used to be the dominant system, but many are looking for even more integrated solutions and some of the biggest

It's quite big in the free software world. Trac used to be the dominant system, but many are looking for even more integrated solutions and some of the biggest (Wikipedia, KDE) migrated to Phrabricator. It's mostly excellent, but know that it's all PHP and it's got a naming convention going that might be considered strange which you can turn off with the config setting serious-business (and that may be the only funny thing about it).