I noticed that Amazon has adopted the tag cloud metaphor in its newest feature known as "concordance". Clicking on concordance gives you a list of the top 100 most frequently occurring words in the book; mousing over each word provides the exact number of mentions in the book; clicking on the word brings up pages on which the word is mentioned.
They are using the simple and elegant presentation that I praised here, the same as Flickr. Beautiful as it is, it took me a little while to come up with a use case for this feature. But I did!
Imagine someone wanting to buy a book on probability for self-study. It is a cardinal rule of book publishing that every text book must be labelled "introduction" or "elementary", regardless of content. But Amazon's concordance is here to help. Here are four books of increasing difficulty (Aczel's Chance, Ross' Probability Models, Resnick's Probability Path and Dudley's Real Analysis and Probability):
Looking at the tag clouds, one can roughly judge the level of sophistication of these books. Below I present them in mixed order.
 appears to be an elementary book that emphasizes the key concepts ("probability", "random", "distribution", "independent") while "customers" is the most interesting word indicating it is perhaps an applied book.  is even more novice as we don't find words like "suppose", "system" and "function" that showed up in . Words like "martingale", "sequence", "convergence" give away  as reaching another level of sophistication. I should click on "oc" and "oo" to find out what these mean.  is the only book on probability where "probability" is not in the top 10; it is evidently entirely theoretical with oodles of measure theory. (So,  Ross  Aczel  Resnick  Dudley.)
How else have you used this concordance feature? Let us know!