Free learning management system with good collection of resources, including discussion threads, assignment sharing/grading, messaging tools, flashcards, color-coded subjects/classes, more.
Really nice collaborative science site. Access resources, partake in discussions, set up virtual classroom spaces, connect to other teachers, scientists, etc.
Site lets teachers post a question online. Students then respond via webcam (no account required) and have their (moderated) comment added to a really great-looking video wall online. Site does cost $65 annually, but that includes 10 walls and virtually endless questions/responses.
Enter a problem, and a unique URL is created. Once shared with others, a group can collaborate by adding questions, comments, solutions, etc. Very easy to use and effective way to solve problems.