“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.”
Related to this is the concept of forcing authors to provide meta without knowing it. This same idea was used by Inktomi in their Content Categorization tool that I talk about here. When an editor put a document in a certain folder — voila! — he just provided meta. Same as when he mentioned a certain term, put his name as the author, etc.