One of my favorite anecdotes in the book was about an errands marketplace.
A group of researchers in Eugene, Oregon experimented with a digital version of the community errands list; in which mobile devices negotiate about sharing tasks such as picking up dry cleaning, buying stamps at the post office, picking up a book at the library.
This is an academic research project, so it includes wearable computers using game-theory-based agent software to negotiate the exchange of tasks, using a system of points accounting for difficulty and distance.
The algorithm may be overkill; one can imagine a simpler, pub-sub, hackable version of this whereby people publish their errand list, and others can click off tasks. Perhaps with an Ebay-like reputation system and security levels if the group gets big enough. Might work for a block association or co-housing group or apartment building.