This is a community wiki post, everyone with trust level 1 or higher can contribute here with new content. Please only add open-source software to the list. Also have a look at this overview of open-source alternatives.
sourcehut.org the open source software development platform - the hacker’s forge!
codeberg.org is similar to GitHub, but Open Source and Free, like GitLab.
- Linux MultiMedia Studio https://lmms.io/