I have been very impressed with the power of Wolfram Alpha as a teaching tool. When I first started using it I would show something in class with it, refer to it and encourage students to check the site out on their own. Since I've discovered Wolfram Alpha Widgets, I've made more explicit use of this great tool.
Last week I was reviewing operations with fractions with a group of students who have struggled with fractions for a long time. Rather than just showing them the operations with fractions and giving them a worksheet, I quickly reviewed the operations and showed them this widget, which will not only perform the operations but will also show them the steps required to get to the answer. I sent them to the computer lab with a small number of questions and encouraged them to work on a problem and then to check it using Wolfram Alpha. My students loved the immediate feedback they received from Wolfram Alpha. One student came to class the next day so excited that Wolfram Alpha was able to help her get through her homework. It's a great tool that adds another dimension to the classroom.
In addition to being able to find widgets you can modify any widget that you find or create your own widgets. I'm beginning to experiment with how to create good widgets that help students gain a better understanding of the concepts I'm covering.