Desmos is a new web service that allows you to create rich educational content that works through a browser so that anyone can access it, no matter what operating system platform they are using.
It is free to sign up and use and even features the ability to collaborate on the content with others.
Desmos includes support for graphics, photos, math tools, and interactive lessons. You can create math lesson using the math tools, including graphing and equations. You can also the graphics program to create interactive diagrams where students can move the labels themselves.
It is easy to use and there is a blog and support page to help you out. It looks like a great way to create some great content for your classes.
Lessons are all licensed under Creative Commons but are visible to everyone.
Check it out.