Indian Creek campground in Yellowstone
I believe this is supposed to be the Indian Creek Campground in Yellowstone National Park (which would put it in Wyoming), so I'm going to put that review here.
This is one of the best campgrounds in Yellowstone. It is seculded, and recently renovated. It is close to Mammoth and everything on the North end, plus most people ignore it's existence. Unfortunately (or fortunately) you cannot reserve here.
Month of VisitSeptember