Ironmaster Dumbbells, Kettlebells
Replace a wall of regular dumbbells with these compact adjustables in a space 2 foot square. Larger sets come with a stand with inbuilt storage. And you can upgrade from the 2 to 20.4 kg (45 lb) set to the 34 kg and 54 kg (120 lb) set for the difference in price. (And go to 72 kg sets; people do this!)
Cost hundreds less than the equivalent in hex dumbbells, and they're nicer to use because they're more compact - there's little overhead clash during presses, for instance, and they're more maneuverable. And they're more of a pleasure to use than bulky and unreliable plastic-dial-driven adjustable dumbbells.
The kettlebells use the same principles, and adjusting from one weight to another happens in smaller increments, meaning you can progress more smoothly and safely.