Adjustable dumbbells have been around for a long time. They are great for people who don’t have a lot of space for too many pairs of dumbbells. Take the PowerBlock Sport 50 Adjustable Dumbbells: they have a compact design and let you train with 10 to 50lbs per hand.
These dumbbells let you adjust the weight up or down with a magnetic polypropylene pin which is 500lbs rated. They replace 8 pairs of weights.