How do you transfer ownership of a Drupal contrib module to someone else? I own the S3FS and Date iCal Drupal contrib modules, though I abandoned development on them quite a while ago, due to moving all my work to another platform. However, I am still on the record as the "owner" of these modules, despite there being other maintainers who have taken over.

There's no obvious way to remove myself as owner. I can't uncheck any of my permissions from the Maintainers listing. There doesn't appear to be any UI that lets me designate an existing maintainer as the new owner. 

What do I do?
You can either:

Open an issue on the Drupal.org project ownership queue, asking to make the new project owner a user on drupal.org that you know be willing to become the new project owner
Open an issue on the same queue, asking to make the new project owner the maintainer who did more commits or who was more active in following and answering the project issues
Open an issue in the same queue, stating you are seeking for a new project owner, using Needs maintainers as Component value

Once you open the issue queue, the procedure will be handled by Drupal.org site moderators.
As alternative, you could simply set the Maintenance status field of the project to Seeking new maintainer and (eventually) the Development status to No further development. Users who need the project, and who are able to develop code for a project, seeing you set that status, could ask to be the new project owner or maintainer, if they are interested.
Finally, if you think you won't maintain anymore a project hosted on drupal.org and you won't use Drupal, you could delete your account on drupal.org. Authenticated user are now able to delete their account on drupal.org, for any reason they could have.