Juvenile science fiction recommendations?

I got a request from Hillary Rettig: those gift-giving holidays (you know, Cephalopodmas and some other religion-tainted days) are coming up, and as we are all pale, text-focused people here, she thought the Pharynguloid hive mind would be the perfect place to gather recommendations for books to infect young brains with the imaginative side of science. So, please, post your recommendations for juvenile science fiction right here. Everything from the classics to the very latest stuff is welcome.