Common Sense Media has an extensive collection of materials for teaching digital citizenship.  They have an entire curriculum that spans across all grade-levels as well as materials for families.  
At Cyber-wise they view digital citizenship as the first step to media literacy and have designed and gathered materials in support of that.  
At Digizen the emphasis is on helping users of technology become responsible "DIGItal citiZENS".  There resources include videos, games, and printable materials.  
Net Smartz offers a variety of materials including videos, project ideas, printable .pdf's, online safety quizzes and even an online educator training program.  
InCtrl has standards based lessons designed to teach students in grades 4-8 about digital citizenship.