One-way text communication with your students. Messages can be sent immediately or scheduled in advance to remind students of due dates or room changes. Phone numbers are not exchanged between teacher and student.
Create a virtual corkboard to which students can add notes (varying degrees of moderation, but students don't have to have account to create notes). Can be used for brainstorming, comments, or questions.
A completely free tool to import videos from YouTube and other sources and add questions and additional info. Student responses and progress are recorded for the instructor.
If your students have access to a computer, you can embed these little 1-question polls or brainstorms to a webpage (like in D2L) for them to contribute answers to.
Students text their answers from their phones. Answers are displayed live as they come in. Free version allows 25 responses per poll. There is also a free plugin to integrate polls with your existing PowerPoint.