Yay, saddle spinners are my favorite piece of playground equipment! We have one here in the office at KaBOOM! and I play on it almost every day. It's a scoop-shaped seat that's free to spin in circles on its post. This post is on a slant, so that as you simply lean over, your potential energy (gravity) is converted to kinetic energy (spin). This also makes you spin faster and faster just by holding still, as long as you're leaning off beyond your vertical axis.
Kids know how much fun spinning is! Even without playground equipment, kids will spin in circles until they collapse in laughter (and dizziness). When I was little, I used to call the foyer in our house "Spin-Around Land." I think that this particular company that makes the Saddle Spinner is really in touch with how much fun and learning a child can gain from rotational motion. I highly recommend it!