Website: http://www.ck12.org/
Free Android Phone app: https://play.google.com/store/apps/details?id=org.ck12.app.practice
Free Android Tablet app: https://play.google.com/store/apps/details?id=org.ck12.app.practice
Google Chrome app: https://chrome.google.com/webstore/detail/ck-12/ilmbnmigihncgeckjgmkehcgkdeohkhl
Free iPad app: https://itunes.apple.com/us/app/ck-12-free-fun-way-to-learn/id909343639?mt=8&at=10l6SU
Free iPhone app: https://itunes.apple.com/us/app/ck-12-free-fun-way-to-learn/id909343639?mt=8&at=10l6SU
Paid iPad app: https://itunes.apple.com/us/app/id584413429?mt=8&at=10l6SU
Paid iPhone app: https://itunes.apple.com/us/app/id584413429?mt=8&at=10l6SU
Paid Android Phone app: https://play.google.com/store/apps/details?id=com.ktlmobileapps.levelitbooks
Paid Android Tablet app: https://play.google.com/store/apps/details?id=com.ktlmobileapps.levelitbooks
Website: http://www.coursesmart.com/
Free iPad app: https://itunes.apple.com/us/app/etextbooks-for-the-ipad/id364903557?mt=8&uo=4&at=10l6SU
Free iPhone app: https://itunes.apple.com/us/app/etextbooks-for-the-iphone/id325230226?mt=8&uo=4&at=10l6SU
Free Android Phone app: https://play.google.com/store/apps/details?id=com.coursesmart.android.coursesmart