The Math Goodies website is one of the finest sites of this nature I have ever seen or used. Lessons are offered and multiple choice questions are provided.
Explanations are readily available for all questions.
I found the questions challenging and the explanations comprehensive and clear. For the teacher and or parent, there are sections for Books, Resources, and Links. For the teacher, especially a new teacher, areas can be found for Interdisciplinary Studies, Classroom Gems, and Real World Connections. I found the last one of great value because Mathematics teachers are always looking for real-world connections in terms that a student can identify with. In conclusion, let me restate that this is a great site.