I've used that one, it has a lot if info for places not on many commercial campground lists.
If you have a smart phone or a tablet, there are also many apps that can give good info. Allstays has a number of good ones, including a pretty good resource about Walmart overnight parking locations.
Camping Apps: iPad/iPhone Apps AppGuide
Also many states have great resources. Oregon is one great one we used, not only can you find and reserve a site, you'll find maps and photos of the campgrounds. Oregon State Parks and Recreation
I only wish all the states were as good! After Washington and Oregon state parks, we came through Colorado and were disappointed in the lack of good info on campgrounds. When we searched for campgrounds it included mobile home parks. Quite a step down from the other resources we'd used to find places for a stop. I was quite surprised, having spent a lot of time in CO I really thought they'd have better info. I haven't looked lately, maybe they've gotten better.