Remind101: Remind101 allows teachers to send text messages (and email) home -- to students and/or to parents -- to offer reminders and updates for class. Remind101 allows teachers to communicate with their classes without either teacher or students having to share their phone numbers.
Poll Everywhere: As the name suggests, Poll Everywhere allows teachers to use cellphones for polling in class. Students text their responses, using their cellphones to give feedback, answer questions, take quizzes.
Celly: Celly provides SMS-based group messaging. Classrooms can use the service to take quick polls and quizzes, filter messages, get news updates, take notes, and organize and hold study groups. The groups can be public or private, moderated or open.
StudyBoost: StudyBoost allows students to study via SMS-based quizzes. The questions can be self- or teacher-created, and can be multiple choice or open-ended.