SpeQ Mathematics - SpeQ is a small, extensive mathematics program with a simple, intuitive interface.
WolframAlpha - WolframAlpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
Qalculate! - Qalculate! is a multiplatform multi-purpose desktop calculator.
Cymath - Free Step-By-Step Solutions to algebra problems.
NumWorks - The graphing calculator that makes learning math easier.
Photomath - Photomath is a mobile app that will give you the ability to test your equations through a simple calculator interface that will fully explain the solution in a step-by-step fashion. Read more about Photomath.