One of my best purchases ever, in hindsight, was to order 10 flatpacked medium-duty storage racks when they were on discount (20 EUR each, 4 EUR shipping for all of them together) and just pile them up at home.

Now I basically have "flat surface" legos; need a storage area? Construct a rack. Need a workbench? Construct a half rack. And they're rigid enough that they usually don't even need wall-anchoring... (with no kids around, that is)

Been constructing things with them for years now, most recent project was a low-budget kitchen extension to have a place to put my appliances (freeing up actual kitchen counter space) with storage crates on shelves below it.