I've been having a nice debate with a commenter about whether Wikipedia is a public good or not.
I don't think it is; I can very easily exclude you from consumption (ie, publishing your microchunk) by editing it out.
So I don't think public goods analyses (collective action, market failure, etc) really apply. I think about it as a two-sided market, rather than as a good at all, but that's a long, boring story.