So for #19, Web 2.0, I tried out Zillow.com, which won an Web 2.0 award for Real Estate. It's basically a site to find properties to purchase... or put up properties to sell... Pretty nifty, actually. You can do a search for a city, or a zip code, or a particular address, and it will bring up houses for sale in that area. It includes a satellite map, which is cool.
There are other parts of the site, including discussion boards and lists of real estate agents.
I don't know how useful this would be in the library, except to recommend to customers.