From Wikipedia via André's agent coordination project:

Stigmergy is a form of self-organization. It produces complex, seemingly intelligent structures, without need for any planning, control, or even direct communication between the agents. [...] By offloading memory to the environment (as stigmergic traces), and computation to interaction between agents and traces, complex distributed cognition is performed by remarkably simple organisms.

Thinking about the aliens in Blindsight.

Maybe an interesting contrast to Smalltalk where objects communicate by messaging with each other.

I guess any sort of collaborative editing of a document or codebase is stigmergic (including, as mentioned in the entry, Wikipedia itself).

Most of the natural examples of stigmergy are spatial though - building structures or pathfinding - which feels a bit different.

Applied to software (and AI) I guess a lot depends on what you define as the agent and what you define as the environment.