musictheory.net. At first glance it can be just a review or a teaching tool. Broken down into content specific lessons, it allows me as a teacher to step by step introduce a specific aspect of music theory. I typically don't teach from this in the classroom, but students can use it as a review resource at home via the Internet or even the smart phone/tablet app.
My favorite part of the website are the exercises. Students can set-up exercises that will help them practice what we have covered. The exercises can be tailored to the student's specific needs and involve both their note reading skills and listening ability. At any point in the exercise, a student can stop and generate a report of their accuracy statistics, time spent on the activity, and a verification code that they can email me. This code allows me to see the report at home or at school at a later date and time.
I currently use this site every few weeks on "Theory Thursday's," but I am brainstorming ideas on how to use this as a more consistent part of my teaching.