Well, I looked at quite a few of the sites as it was fascinating and I couldn't choose. I first looked at propsmart. That was terrific. It's is supposed to be a monster site tht has almost every property for sale on it and searchable. Not just ad free but people can also blog about the properties and add comments to those properties... interesting. That site took be to bookcrossing.com which was fascinating. Take a book you loved. Register it with bookcrossing.com write the BCID (identification number) in the book, and then... release it into the wild and basically wait for someone to pick it up, read it and then log on to Bookcrossing and add their comments and PASS it ON. Take a look. It's fascinating! Also had a look at ETSY.com. amazing style to this site, and what a great idea, an auction site for handmade/homemade things! excellent.
As for usefulness in a library, I can see that sites like propsmart are trying to make things easier to go to one place on the net to find everything. It's all good.