Here are four free online science textbooks which all have lots of interactives that I’m adding to The “All-Time” Best Science Sites:

CK-12, which I’ve described in a previous post (see “CK-12” Has Free Resources In All Subjects & Individual Student Progress Can Be Easily Tracked).

Science Book

Open Educational Resources from UEN, which also has a separate page for online science interactives.

Scott Foresman Science