"The Computational Knowledge Engine." Simply type an equation into Wolfram|Alpha, say, 3x^2 +4x +1 = 0, and Wolfram|Alpha does the work for you. Wolfram|Alpha also has a dedicated educators page with lesson plans, classroom resources, and tips for using Wolfram|Alpha in the classroom.
Mathalicious offers free resources to teachers that allows them to relate mathematics to students' by applying it to real-world problems. Resources range from middle school maths to Algebra I and II, in subjects like fractions, decimals, probability, statistics, graphing, and more.
Brightstorm hosts thousands of free, math-related videos on topics ranging from Algebra I to Calculus. The videos cover the topic and work several sample problems for each. The site charges for some elements, but all of the math videos are free and organized into subject and topic and are completely searchable.
Math Live has interactive math lessons with resources for parents, teachers, and assessment, as well as an interactive glossary. The cartoon-like, television interface is perfect for younger students, and the fact that it's HTML5 means it's mobile-friendly. Lessons range from place value to basic geometry to probability.
NASA's educator website offers free classroom activities, lesson plans, educators guides, and more for almost 300 space and science based mathematics topics. Science teachers will love this one, too!
Mr. Reddy's Geometry Toolbox
This one's fun. It's a flash-based white board with geometric drawing tools. It includes a compass, ruler, protractor, line tool, and free hand tool, as well as a variety of colors.
The US Federal Government offers tons of free resources for teachers of all subjects. The math resources stretch across a variety of subject areas, including Algebra, Geometry, Data Analysis, Mathematics Across the Curriculum, and more.
Thanks again to all of the people from Classroom 2.0 who offered suggestions for mathematics resources. I hope you find something here, as well.