They are commonplace in my experience, and although the designs vary, they all follow a common theme in that they refer to children. The one shown above translates as "VOLUNTARY - 30km[/h] - because of us".
German residential streets frequently have 30km/h speed limits, but signs like the one above are typically seen on the approach to small communities, and appear to have been erected by the community (some even appear, from a distance at least, to be made from plywood). Clearly it cannot be enforced, but equally clearly someone has gone to a lot of expense to erect this sign by the side of the carriageway. Does that mean it is legally placed, or is it a case of a local community daring the local authority to remove it? I always follow the wish of the communities that erect these signs and limit my speed as requested, but is there any significant evidence that they are beneficial? Is there anything else anyone can tell me about these signs?