This is by no means an exhaustive list, but the following tools are the most often used in my class.
I use Remind 101 to text message homework to my students' phones or to their email accounts. This is a great tool to 'remind' students of virtually anything.
Teachers never see students' phone numbers. Students never see the teacher's. Another added benefit is that parents can join and also be notified of homework.
Google Docs is a fantastic tool that will marginalize the "I forgot my work at home" excuse. Google docs is a browser-based software-suite that mimics Word, Excel, and PowerPoint. Docs also provides the user with a drawing application and survey creator called 'Forms.' The best part of Google Docs for me, is that my students can "share" their documents with me and we can track all changes and edits. For the teacher, docs also aids in helping student manage their time, as all work is time stamped.
Weebly for Education
I use Weebly for my class site and for my students' blogs. It is super -easy to use, and I can set up all my student accounts in advance and monitor them all.
Ning the Social Network creator
Ning is used in my classes currently for a "Romeo and Juliet meets Facebook" project.
It is a safe, controlled social network that I own and control. No one from the outside can "get in," but my students can still have meaningful interaction outside of the classroom.
This is a great vehicle for virtual Socratic Seminars and essential question discussion.
Poll Everywhere is yet another fantastic tool that has become a student- requested activity. My students love using their cell phones in class. With this tool, I have nearly 100% class participation with each review.