Use these websites to find places to sleep overnight for free
Free camping websites
These sites will help you search free places to camp or sleep overnight. This may include urban settings like Wal-Mart parking lots.
Free Campsites: freecampsites.net
While not designed to help you find free campsites, these are two commonly recommended apps for finding campsites.
iOverlander is an app, but also allows campsite searching through their website. this is not necessarily for free campsites, but it may be useful in finding campsites.
Wikicamps is an app. Their website doesn’t provide any functionality other than discussing features of the app.
Notice: I have no financial or other relationship with any of the services listed here. I just like camping and I like free stuff.