Over Labor Day weekend, we camped out at the Elbert Creek campground - it's right after the Elbert trailhead and right before the Massive trailhead, and on the way to the Halfmoon/So. Halfmoon creek trails.
Anyway - the camp host there told us that with the coming and going of Labor Day, all of the established campgrounds in Colorado become FREE - at least those in national forest areas. The only catch is that facilities (i.e. bathrooms, water pumps, etc.) are no longer available. Even so, that seems like a pretty good deal to me. We were right on Halfmoon Creek, with a great view of the snow-capped Mt. Elbert from our tent.