The following sites contain open textbooks in a variety of subject areas, but the links below will take you directly to the textbooks about economics. This is by no means an exhaustive list!
Bookboon.com contains free eBooks and textbooks that can be downloaded as a pdf; no registration needed. These books cannot be altered, but can be freely printed, shared, or embedded in non-commercial ways. They are intended only for individuals, colleges, and schools.