Here are the ones, that we use the most:
Park4Night and Stay free App: Besides displaying free spots to stay within their maps, they also have other useful entries that show public facilities like drinking water, showers, toilets and laundromats. You can also find campsites within these maps, which you can directly call. We have often done this to ask if we can just come and use the shower or wash clothes. Most always say yes, and only charge very little to use their facilities.
Google: Using the satellite view has helped us find quiet parking spots in the woods or at the beach, which had not been added within the open maps yet. When wanting to stay in cities we check Google Maps for local sports centers, as these often have a lot of parking spots that are rarely full.
Local trucker apps: Truckers need many of the same facilities and often these can be used by the public too. These truck stops are mostly near highways and bigger roads.