Curated list of open source and low-cost bug and issue tracking software (2020)
Photo credit: John Schnobrich (Unsplash license)
Edit 1: There are changes for OTRS in 2021. On 1 Jan 2021, OTRS 6 community edition would not have new security updates. Also, the GitHub repository for OTRS 6 community is archived. So, there is no official support for OTRS 6 community edition any more.
There are two forks that I am aware at the moment: Znuny and Otobo.
Edit 2: Phabriactor stopped being maintained after June 2021.