Some Finnish RPG scene enthusiasts have been running an open-source RPG scenario library since 2013. The library contains some Creative Commons licensed scenarios in English, too, but that is not the focus. The contents are mostly Nordic-style freeform.
The Forge & Illusion project originally started in 2002 as a character sheet utility(!) but developed into an online tool to help RPG authors to collaborate. The scenario library is an offshoot of that project.
Free resources for Dungeon Crawl Classics, including adventures:
This page is a more human readable version of the Google Sheet put together by the DCC community. You can use the buttons on the side to jump to different sections and filter to see specific contributors