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.
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I loved this! I find it’s especially interesting for comparisons. For an easy comparison demonstration just put in the names of two different countries.
It is excellent Judy I agree, my students are using it more now to check Maths.
I am about to teach equations to Year 7 (11-12 year olds) and I will certainly show them Wolfram Alpha – perhaps after they have tried some themselves!