Amazon is integrating user tagging into product pages (see image below).
Tags are public by default and can be managed under a “your tags” area that I am failing to find. You must first select a “Real Name” (odd choice of names given the old company called RealNames). Once you’ve signed up and started adding tags, you can delete them or make them private in the management area.
Amazon tags will make it easier for you and others to find relevant content. I wonder how they are going to handle spam tagging and other bad content, though?. Another, possibly more interesting feature would be to allow publisher tagging. The tags would likely be more relevant (and spam easier to track).
See Richard MacManus for more on Amazon Tags.