...is in terms of attention: The only factor becoming scarce in a world of abundance is human attention.
As Nobel-winning economist Herbert Simon puts it: "What information consumes is rather obvious: It consumes the attention of its recipients. Hence a wealth of information creates a poverty of attention." Each human has an absolute limit of 24 hours per day to provide attention to the millions of innovations and opportunities thrown up by the economy. Giving stuff away captures human attention, or mind share, which then leads to market share.
Following the free also works in the other direction. If one way to increase product value is to make products free, then many things now free may contain potential value not yet perceived. We can anticipate the eruption of new wealth on the frontier by tracking down the free.
In the web's early days, the first indexes to this uncharted territory were written by students and given away. The indexes helped people focus their attention on a few sites out of the thousands available. Webmasters, hoping to draw attention to their sites, aided the indexers' efforts. Because they were free, indexes became ubiquitous. Their ubiquity quickly made them valuable (and their stockholders rich) and enabled many other web services to flourish.
What is free now that may later lead to extreme value? Where today is generosity preceding wealth? A short list of online candidates would be digesters, guides, catalogers, FAQs, remote live cameras, front page web splashes, and numerous bots. Free for now, each of these will someday have profitable companies built around them selling auxiliary services. Digesting, guiding and cataloging are not fringe functions, either. In the industrial age, a digest, Reader's Digest, was the world's most widely read magazine; a guide, TV Guide, was more profitable than the three major networks it guided viewers to; and a catalog of answers, the Encyclopaedia Britannica, began as a compendium of articles written by amateurs--something like online FAQs (Frequently Asked Questions).