There are actually so many choices for science lessons. I looked at about 30 of them and narrowed it down to these. I based my choices on what I thought were the most helpful & user friendly sites. I’m excited to share these with you!
How Stuff Works 
How Stuff Works  and How the Body Works  were lifesavers for me in 2006. I was practically useless for anything physical when I had a few herniated discs in my neck. My kids, then in 3rd, 1st, and K. had a list of topics they wanted to learn about that year. So, while I was recovering, they were exploring the various items on the list & accomplishing more than I ever thought we would.
cK-12.org  is a company that started up with the intent to reduce textbook waste. These e-books are called Flexbooks. You get the student book, workbook, Teacher Edition, and the Answer Key for each subject. (Note: You have to email a request for the answer key.) You can get the books in .pdf, Kindle and iPad editions are also available. It was easy to register & download the Earth Science course. It looks easy to navigate and teach from the teacher’s edition. To me, this is something people might pay a good amount of money for, but it’s FREE!
NASA  and IMAGERS  have provided three thematic units . Within each unit, you will find lesson plans, reproducible worksheets, visuals, and links to useful resources. The adventure begins with the online book, Echo the Bat . The teacher’s guide will provide you with printable worksheets, discussion questions, and more.
The Vega Science Trust  (out of England) has a series of videos and articles involving, well… science! If you want to learn how to generate electricity, click here  to watch the video, but also click here  to get the article that shows you the lesson. There are various lessons to get your student interested and ready to learn. Be sure to see what other lessons on video  are also available!
Math Science Nucleus 
Math Science Nucleus  has curriculum for both Elementary and Secondary students. There is a teacher section, as well as interactive components for students, including story books, multimedia activities, slideshows, and online tests.
University of California  offers UCPrep for students as well as teachers. The link will take you to the page of lessons where you will find video lessons for each course from UC College Prep’s Classic Collection. They have more than science. There are lessons for math, history, and social sciences as well.