Amazon.com now allows you to place "tags" on any product. From their "What's This?" description:

Tags provide an easy way for you to "remember" and classify any item on the Amazon site for later recall. For example, you might be shopping for holiday gifts and tag items with the names of the people on your shopping list. At any time, you can review the items you've tagged and make final gift decisions. You might also want to tag items you've purchased in the past. This allows you to organize your collection in any way you wish. You can also use tags to help you remember items you're considering for purchase, e.g. if you are planning to buy a digital camera, you might tag several makes and models so that you can look at them side by side, organized the way you wish. Don't like the way Amazon has organized its science fiction or cookbooks? Tag the items you care about with the categories that matter to you. Because people's tags are (by default) visible to others, a great side effect of tagging is that you can navigate among items through other people's tags. What items have people tagged "gift" or "Tuscany" or "robot?"