NASA is a great resource for educators with literally hundreds of sites and resources available for free. Another one I just found is NASA 3D Resources. The 3D resources page has 3D models, textures, and images from inside NASA. All of these resources are free to download and use.
The 3D models can be used in almost any 3D software package and include the ISS, satellites, spacesuits and more. This could be a great resource for STEM classes, especially design, CAD, and engineering.
Voyager Image of Jupiter
There are also some cool simulations that would be great for use in education.
NetworKing is a game that allows you to take on the role of Network Manager and puts you in charge of building up a communications network that you will use to support scientific missions.
Station SpaceWalk is a simulation of doing a space walk to conduct repairs on the International Space Station.
There is an interactive demo of the Space Communications and Navigation (SCaN) system that shows how it provides communication services for NASA missions.
All of these are great educational experiences for students. Students learn about STEM subjects, get to think critically and solve real world problems.