Campendium has a handy and free app, in which you can search for specific criterium (such as 'free').
Freecampsites.net is a more difficult to use website (on a phone at least), but has a wonderful collection of free places to sleep for the night.
The majority of these free areas are in national forests where dispersed or roadside camping is almost always permitted. They will also often list rest areas and Walmarts (and similar) that allow overnight stays.
Both of these services were helpful when we were traveling through Colorado, and public land (with free camping areas) was plentiful. Closer to home, public land becomes sparse, and high dollar (often busy and/or close to the highway) campgrounds are often the only option. I found two pay services that helped keep us away from the busy campground scene.
'Boondockers Welcome' is a service which, after paying your annual fee, connects you with people all over the country who wouldn't mind you parking on their property for a night. You must have a self contained RV (not needing any hookups or restrooms), but the annual fee is fairly reasonable. I searched a few locations we were planning to travel, and found many of the overnight options were in driveways or very near homes. This wasn't the experience we were going for, so we opted against this service.
'Harvest Host' is and annually paid service similar to Boondockers Welcome, only rather than private residences these are businesses. The idea started with farms, as they often have plenty of land for RV overnighters, but expanded to wineries, breweries, distilleries, and attractions.
For an annual fee, you get an app and website to put you in touch with thousands of cool places to stay. They do ask that you make a purchase from the location if possible, but that isn't a problem for us. If it is a winery or brewery, we often buy a tasting and generally a bottle of our favorite. Many of the locations have a restaurant on site, which means we come come, have drinks and dinner, and spend the night on the property. This has worked absolutely wonderful on a couple of our trips, and we've even spent several weekend around Ohio doing quick overnight trips to wineries and breweries nearby for quick escapes from daily life.
One feature we've found about our Ohio wineries and breweries is they often have live music on the weekends. This adds the the experience of a quick weekend getaway tremendously, and only costs us the cost of dinner, drinks, and a little bit of gas. It's extremely tough to beat!
I foresee us keeping our Harvest Host membership for years to come, and trying to check off all the cool locations around our home on weekend trips as well as utilizing the local spots as much as we can while traveling. It seems every time I log on to the app, the amount of hosts increases.
As of this writing, we've stayed at 12 Harvest Hosts in 6 states. Some of our locations have been better than others, but we've never had a regrettable stay. Almost always there is at least one other camper, but we've always had plenty of space.