Site offering a variety of apps that teachers can use to create activities and that students can use on their own. Intended for EFL, but would also be useful for other languages in some cases.
Limitations: As of 6/9/2016, no new registrations are allowed, although the site can be used without registering.
Upload videos or link to YouTube. Add video, audio, or text comments at the exact place in the video where you want it. The creator can also add buttons for simpler comments, such as "awesome" or "I don't understand."
Limitations: As of 6/9/2014, the audio and video comments don't seem to work.