WolframAlpha – this is definitely worth taking a look at if you have not seen it before.
For example type in:     solve 3x+7 = 13
(or any equation of your choice, use x^2 for example for powers!)
Note that you have the option to view the steps when the results are returned.

Have a look at the gallery of examples for inspiration, including Mathematics.

Update 12th April:

Recently introduced Wolfram Alpha for Educators has examples of how it can be used in the classroom, note the Wolfram Alpha blog post on Step-by-Step Math and the Mathematics Lesson plans.