Described here : http://www.stigmergicsystems.com/
Stigmergy is roughly "communication through leaving cues in the environment" as with chemical trails left by ants.
JohnRobb now using this idea wrt the BazaarOfViolence : http://globalguerrillas.typepad.com/globalguerrillas/2004/07/stigmergic_syst.html
In BiologyAndComputing, SteveBurbeck points out that multi-cell organisms are based on StigmergyStructures. Essentially, the cells are temporary; bodies aren't just a bundle of living cells, they're a bundle of ephemeral living cells which are connected and communicate stigmergically through the non-living persistent structures (eg. bone) "we're termite mounds" (or corals?) ( (WarpLink) PersonalIdentity )
But then, what communication isn't stigmergic? Writing must be - ie. we leave writing around the environment. It seems like speech shouldn't be. Is there a MediaAndMaterials question lurking here? Eg. that some sorts of communication are "more stigmergic than others" and this can bias the overall system behaviour in certain ways? (Hey, I claim BonusFreakyConnectionPoints for this ... although that was actually prefigured by a link to WatchingAnts already on MediaAndMaterials, amazing!)
Spiders outsource cognition to their webs.
See also :
Added 2020-05-15 : Originally 2017-11-22