comment fics are fics written in response to a prompt and posted in the comments of the post. I've been over at comment_fic on LJ, filling some prompts. There are daily prompt posts where people leave prompts to be filled, and anyone can fill whatever prompts take their fancy. The community is supposed to be mirrored here, but the daily prompt themes haven't been posted here forever. There are probably other comment fic type communities. I used to write for fic_promptly, could still go back there and fill some old prompts, but no new prompt posts have gone up in years.