There is a GitHub organization to hold open-source extensions and extension developers:
Also check the individual members of the organization for extension sources. For example, both me and Andos have our extensions open source on our GitHub accounts instead of in this organization.
Some extension sources are also available here, but I am not sure if they are meant to be open-source or not or with what license:
The idea is that extensions which you want the extension development community to help with can be in this organization, or if an extension developer does not yet have a GitHub account but their extension is open-source. Ideally extensions can be maintained by other extension developers, not just the original developer and/or Clickteam.
If you want to open-source your extension, create a GitHub account and post in this thread to be added to the organization. You can create your GitHub repository in your GitHub account and optionally transfer it to the organization, but you don't have to.
If your extension is already in the organization and you want it to be removed or transferred to your GitHub account, you can ask for that too.
Note that you don't have to be a member of the GitHub organization to contribute - all you have to do is fork a repository for an extension you want to work on, make changes, and then submit a pull request (you will need to learn about Git and GitHub, of course).
Ideally extensions sources should not contain the entire SDK they used, instead it should be mentioned which SDK is used to compile the extension. It is also good to add a license to the source in a file called LICENSE, otherwise the source is subject to implicit copyright and thus we're not very sure what you want others to be able to do with the source. Best case scenario is to expose the source to public domain if possible via unlicense, but the choice of license is up to you.
As of writing this post the main issue is missing licenses - many of the extensions do not have a license and so we're not sure if they're meant to be fully open source or with what terms. Ideally they should remain open source so they can be updated as needed, but there is no guarantee of this. Entire repositories may be deleted if the author or Clickteam requests it.
As of writing this post, Looki, me, Villy, Francois, Yves, and Andos are administrators of the organization. You can view public members here but keep in mind that each member can choose whether they are visible as a member or not, and we have no control over this.
Thanks for reading this entire post very carefully, and remember I won't be able to edit it anymore to keep it up to date with the latest information.