BrainPop: Short, simple, animated videos that could give a basic level of understanding when you're starting out.
ScienceFlix: Science focused database, offering information on a range of topics. Topics will typically be available at different reading levels.
Britannica School: You can choose the appropriate difficulty level and search for any topic.
PowerKnowledge: Divided into Earth and Space Science, Life Science, and Physical Science, it offers short, organized text with embedded videos.
World Book: An online encyclopedia designed to cover major areas of knowledge, with particular strength in scientific, technical, and medical subjects.
Rosen: various texts shining a spotlight on topics such as Earth Science, Ecology, Global Issues, and Natural Disasters.