Here I started to collect URLs and APIs of existing CKAN instances. I came across the dataportals.org portal which provides a comprehensive list of Open Data sites and portals. However, there are just about 50 CKAN portals in this collection where some of them are down and a lot of APIs are missing.
Using a short script I harvested the dataportals.org CKAN portals and merge them with my list. Before adding new portals to the list I check if the URL (and the API) is accessible (performing an HTTP GET request).