I've got to clear these damn browser tabs.
Anchor Stone Blocks were created in Germany in the late 19th century. They're red, yellow, and blue cast stone, designed to mimic the brick, limestone, and slate, respectively, of traditional European architecture. They were crazy popular for decades. then Germany got on the wrong side of a couple of wars, and the Richter's Anker-Steinbaukasten factory got on the wrong side of the Iron Curtain.
But nowadays they're back, and there's a small Anchor Block otaku community devoted to their appreciation. You can appreciate them, too, for about $150-200 a set. New blocks and vintage blocks all use the same dimensions--in some cases, they even use the same molds--so scrounging around for old sets may be a more interesting option. ahh, ctrl+w.
Anker Stones - Manufacturer site, mostly in German [ankerstein.de via dinosaurs and robots]
Deep fansite: Richter's Anchor® Stone Building Sets [ankerstein.org ]
Anchor Stones Construction blog [rekna.blogspot.com]
Check out various sets of Anchor Stone Blocks, including one of the Froebel Gifts, at Amazon [amazon]
Search eBay for Anchor Blocks, fewer than I thought, but cheaper [ebay]