Joseph Kerski has recently shared an app by Nicolas Holm listing the open data portals of the world. This app will undoubtedly help you in case you're working across jurisdictions, making it easier to find data sources.
However, one might wonder how feasible it is to keep all the information in the app up to date. A way of addressing it could be to rely on local stakeholders and crowdsourcing. Still, the effort is worthwhile and will challenge us to consider other actions to facilitate access to open data portals.
Note: want to know more about the tool used by Nicolas to create this app and even create your own? Check out the ArcGIS Web App Builder documentation.