Metadata is expensive.  While management of metadata is just as important as generation, I will focus on the creation, or more precisely collection, of metadata because you can't manage what you don't have.

Most efforts to obtain metadata focuses on the creation phase of information using forms, tags, and inferences.  Unfortunately, creating metadata this way impeds creation of information by degrading user experiences and increasing knowledge requirement.  Emergent Markup Languages addresses some of the issues, but not significantly enough and not at all when forms are used.

A powerful way to generate metadata is by donation as the title of this post suggests.  By donation, I mean consumer of information (i.e. user viewing a webpage) voluntarily contributes metadata about all or parts of the information on the page.  As donated metadata accumulates over time, value of information goes up.  This technique can be applied to only certain types of applications, but I believe it can be a powerful tool for generating metadata cheaply.

How to encourage such contribution is a HCI problem and can be discussed in another post some other time.