Interactive pricing guide. You can also buy or sell things here. It would be helpful when learning about budgeting money. Percents, and sales tax could be integrated easily. A good place to collect data.
You can search different cities and find good deals on food. This would be a fun website to use when teaching about money or sales tax. Students could make a menu or figure out how much they can buy given a set amount of money.
Web site for classified ads. Some statistics: used by 50 million people, doing 12 billion page views per month, 8th most popular wiki for English language page views.
This site can be used to organize your books. There is access to Amazon books, ratings and sales ranks. You can check books' availability in over 300 libraries in 11 different countries, check out what's in your friends' bookshelves and view books previewed through Google Books. You can embed your bookshelf on your blog.
BookJetty is a social utility that connects you with your friends' bookshelves and checks books' availability in the libraries. As a teacher this would be beneficial to share knowledge with other teachers. Also, being able to check availability of a book at one site would save time.