Many faculty members mentioned difficulty in finding Open Educational Resources according to a recent Babson Survey. This has prevented them from implementing them in their courses, along with the time required to do so. There are many different types of OER, from single images that are licensed for re-use to full courses with accompanying textbooks, multimedia, and assignments. Each involves a different level of commitment, from creating your own resource from scratch, to combining existing resources in a way that meets your needs, to simply adopting an open textbook as is.
The idea of combining many different types of freely available materials into a single course-specific "textbook" is rather daunting. These products and services facilitate that process to make it more manageable.
These searchable collections allow educators to find a variety of materials in different disciplines. Most offer advanced searching options that allow you to narrow what can sometimes be an overwhelming number of results into a manageable list of quality resources. OER Commons and MERLOT are the largest repositories with broadest coverage; the others are more targeted.
These collections are focused on one discipline, but still provide a wider variety of content than an open textbook on a single topic.