Patterns for Algebraic Thinking
Number Cruncher -- Students can easily see patterns, using a two-column table and when they know the machine's rule, they can type it in and check their answer.
Function Machine -- Students must create their own tables to record inputs and output.
Shape Pan Balance -- uses a balance scale to compare the weights of different shapes.
Patterns in Math: People Patterns -- See if you can extend the row of smartly dressed people -- chose your level of play. Then have students share the reasoning process.

What a Crowd! practice estimation skills.
Estimator Four uses interactive games to practice estimation.
Estimator from the Shodor Foundation for online math tools.
Comparison Estimator from the Shodor Foundation to compare geometric figures.
More or Less Estimator 
Professional Development Opportunities for Mathematics

Illuminations -- NCTM
Lessons for Project Interactive
A Problem Solving Rubric
Math Archives is an extensive collection of activities and software for teachers.

Elementary Math Web Resources

National Library of Virtual Mathematics -- you've got to see it, to believe it. NLVM

Office of Mathematics Education
Elementary Math Specialist 
Melanie Jenkins
Secondary Math Specialist
Kristen Heaney