As part of The Geometry Forum at Swarthmore College, PA, this great site is a math question-answer forum. Students of all grade levels write to Dr. Math to ask questions about any math topic of their choice, including how-to's, explanations, and requests for solutions to problems.
Teachers also write and ask for help with topics and certain approaches for the classroom.
Also, a collection of previously-asked questions and answers on a vast number of topics from the elementary, middle, and secondary levels is available for anyone who wants to browse. If you or your students are fascinated with prime numbers, you'll love this site. It gives a variety of information about prime numbers and the largest primes that have been generated. This site is a good reference for teachers and suitable for students from about grades 6 and above. The Largest Known Primes page is sponsored by the University of Tennessee at Martin.