In case you didn't know it, NASA has a lot of different websites, each one covering something different. Which can be a pain when you are looking for something for your classroom to with NASA. The good news is that NASA Education Enterprise is a sort of umbrella that covers all of the individual websites, which is why it is such a good website for teachers and students. A great place to start is the search box, this will search across all the NASA websites for your subject of interest.
On the left side navigation is the link for educators which is then broken down into grade levels -- fantastic when you are looking for lesson ideas.
Also on the left is the link for students which is also broken down by grade levels. I have used this during free time and the students love it.
One of the important part of teaching students to use school technology is of course research, this website from NASA is great for it. I remember the day when I started the students on this website and sent them after as much information on the two robots on Mars; Spirit and Opportunity. I was amazed at what they found.