I love that this assignment finally let me explore Khan Academy - I've always heard that it was such a great source for kids, but never had the time to play with it. I especially liked the math lessons and thought the pretests were insightful (if just a bit too tough for the elementary crowd) and would be useful to students. I found these resources at https://www.khanacademy.org/.
After going through the websites listed, I thought the easiest and most organized one was the OER Commons (www.oercommons.org). I specifically liked their science and technology ideas and found some great NASA lessons that would work wonderfully with our 3rd grade's NASA Virtual Field Trip lessons. I enjoyed the 3-2-1 Liftoff lessons and Amazing Space. They would be a good introduction to all things NASA and the students would have a greater basis on information before talking to the NASA educators. They are located at this link - https://www.oercommons.org/courses/3-2-1-liftoff-01-station-information. I've used other NASA lessons before and they are always accurate, up-to-date, and fun for the kids to use.