books > Crowell and Shotwell, Problems in Introductory Physics
This book is a collection of homework problems for use in an introductory physics course. There is a complete set of ancillary materials, including solutions and an online answer checker. If you're an instructor, you can use these problems as a plug-in replacement for the ones in a commercial textbook, thus insulating yourself from common hassles associated with using the problems from a commercial text. For example, you can change books without having to redo all your problem sets, or you can tell your students that they can use any edition of a particular text.
For licensing information, please see the copyright page of the pdf version of the book, and the photo credits section in the back of the book.