One of the most famous parks in the United States. Most of the rock through out the park is not good. There are a few places to climb spaced out in the park, but are very hard to find. Check with the park officials first about climbing, especially if your going to be doing some lead climbing. Finally, be on the look out for bears in the summer.
While there are only 7 routes on MP for Yellowstone, I've been there a couple of times and was just there last week and there's LOADS of potential. There are boulders and river cliff lines by Obsidian Cliff, boulders in the Hoodoos, and stuff along Lake Yellowstone. And that's just stuff near the road or a short hike away.
Does anyone know the name and grade of the high boulder problem above the pullout road in the Hoodoos? I think it's somewhere around v7-v8. I did this several years ago and I thought it was really good. The landing zone is right on the pullout road and it's a good idea to have a spotter/blocker direct cars away as you climb.