Text Message (SMS), Twitter, and web Polls and Voting, Audience Response System
Now includes presenter tools so that you can create polls on the fly (during a presentation).
Can embed live poll results in a PowerPoint or Keynote presentation.
Limitations: Free version allows up to 40 responses.
Students can receive messages to their phones (SMS) or other messaging clients with multiple-choice questions.
Limitations: Does not seem to support non-Latin characters.
This website will email or text you at designated intervals to remind you of whatever text you choose. Good for learning vocabulary.
Limitations: You need to install a bookmarklet to use this. When you use the bookmarklet, it appears not to work, but it is actually working.