One of the major problems in the mathematics curriculum in the US lately, is the fact that students get introduced to calculators too early and are often overexposed to them.
In Russia, and other countries that were part of the former USSR, students are only introduced to calculators in their late elementary education. Also, even after that, students are expected to use the paper-pencil algorithms...
Here is a website with helpful math applets that I recommended to students before:
Also, for exploring geometry, I recommend a program called GeoGebra. I'm sure that many of you are familiar with it.
You can download it at http://www.geogebra.org/cms/.
Feel free to leave comments on what you think.