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.
For presentations with a difference have a look at the work of Sacha Chua.