Collaborative writing (1), movie making (2), terrorist hunting (3), and volunteer forum moderation (4), are all things. Fanfiction (5) is also written for free.

All of these have been, in the past, variyng degrees of professional.

1. https://www.makeuseof.com/collaborative-fiction-sites-improv...

2. Iron Sky (2012), Moon (2009), Norwegian Ninja (2010) all involved free collaboration, to varying degrees.

3. https://www.youtube.com/watch?v=OR6epSP_Xlw

4. https://dl.acm.org/doi/pdf/10.1145/3491102.3517628

5. http://www.fanfiction.net/s/5782108/1/Harry_Potter_and_the_M...