Arduino (http://www.arduino.cc/) : open-source electronics prototyping platform based on flexible, easy-to-use hardware and software [en].
ExpEYES (http://expeyes.in/) : A tool for learning science by exploring and experimenting [en].
ProtoSnap (http://wiki.laptop.org/go/ProtoSnap_Contributors_Guide) : Arduino-compatible development platform for teaching the basics of real-time programming [en]
Raspberry Pi (http://www.raspberrypi.org/faqs) : a capable little PC designed for kids all over the world to learn programming [en].
[cc] Documents in Creative Commons (http://creativecommons.org/about/) [at least for some of them].
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