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 started with a pair of stage 1 Pro EXP Powerblocks where each dumbbell goes up to 23 kg, now upgraded to stage 2. They can be expanded with stage 2 and stage 3 add-ons, up to 32 kg, then 41 kg. 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.
I'm also using the straight bar add-on, with adds 11 kg and converts the dumbbells into a barbell.