Powerblocks are compact dumbbells that can be adjusted like Russian dolls. Each dumbbell is made of stacked steel plates you can add or remove by inserting a selector pin at a position depending on the weight you want.
Weight increments depend on the model. Some can be micro-adjusted with small weighted cylinders added at the bottom, and expanded with add-on kits.
While this kind of dumbbell is far from cheap, it's in fact a great investment compared to non-adjustable dumbbells. For example, a stage 3 Pro EXP set replaces 28 pairs of dumbbells, which saves not only a lot of money, but also a lot of space.
I'm using a pair of stage 1 Pro EXP Powerblocks where each dumbbell go up to 22.3 kg. They can be expanded with stage 2 and stage 3 add-ons, up to 36.3 kg each. I still haven't decided if these dumbbells are massive bulks of raw ugliness or beautiful examples of efficient design, but they sure do the job.