Free learning management system with good collection of resources, including discussion threads, assignment sharing/grading, messaging tools, flashcards, color-coded subjects/classes, more.
This youtube video explains the "deal" with Diigo and how it can be used. This is excellent for researchers and educators. (You could even use this for grading!)
Create online quizzes in multiple choice, true/false, multiple response, fill in the blank, word list, and essay formats. They are graded automatically, and reports instantly available on student results.
Games for grades k-6.
"We've thoroughly searched the web for engaging and educational games for kids, and we've collaborated with top educators to ensure that all of our games are age-appropriate, relevant and compliant with academic standards. Our team of educators has also created a vast library of "Homework Help" as an additional resource for students, teachers and parents."