Discord/Guilded: Revolt, Matrix is also an alternative if you want a distributed system that can access other instances.

Twitter/Gab/Minds: Fediverse

Mod/File distribution: Forum Software, phpbb can be used though it may require extensions.

Art Hosting: None, but Booru software is a stopgap.

Chan sites: There is a ton of software that can be used for this such as vichan.

Reddit: Voat's codebase can be used, edit: the .win network is closed source.