The following are websites that can be beneficial towards teaching mathematics. I have found use in many of the following websites and they may be useful in your classroom as well!
Khan Academy: A free, world-class education for anyone, anywhere. Khan Academy is a non-profit educational organization created in 2006 by Salman Khan with the goal of creating a set of online tools that help educate students. The organization produces short lessons in the form of YouTube videos.
Discovery Education: This website contains a lot of great resources for the classroom. The best part of this website is that it can be used for any subject area, not just math! This website contains projects, videos, and other resources that can be used within any k-12 classroom. I use discovery education for some of the videos for viewing in the classroom. There are a lot of quality videos that tie up the units that we have covered throughout the school year.
http://www.discoveryeducation.com/
Figure This: This website is fully endorsed by the NCTM (National Council of Teachers of Mathematics) and was created to foster math education outside of the classroom. This website was made so that families can log onto a website outside of school and work on some of the very key concepts that the students are learning within the classroom.
Math Cats: This website contains a lot of projects and interactive tools that help foster math education within the classroom. I have used this website for review games and resources when concepts need to be reinforced throughout my classroom. The students also like this website for the interactive help and projects that the students can use.
Basic Polygons: The following website was developed by myself, intended for any students from 4th-7th grade students studying geometry and different polygons.
The Math forum: This website is a great tool for teachers to collaborate with their peers to see how other teachers are instructing their students. Students can also log on to this website and share their experiences within the classroom and explain what works and what doesn't. I have taken a few math projects from this website and I feel that it is a very beneficial website.