Concrete blocks are very valuable, in any shape or size. We have bought whole truckloads of these, but still more are needed.
Urbanite, the trendy name for used concrete is also a useful building material.
Concrete cylinders, like they use for testing concrete when building bridges, these are available in various locations and always welcome. While we do go and pick them up, ourselves, if you are coming for a visit and know where they can be had, a load of these is very welcome. We are using them, with a little added concrete to put a floor in the barn. They also make the floors in many of our out buildings.