http://www.wolframalpha.com - Mega knowledge engine. Good for checking work and trying things out. Not free for the step-by-step solutions.

https://www.fxsolver.com - alternative to Wolfram Alpha for solving math and engineering problems quickly. Large library of equations to check, edit, calculate and plot.

https://www.cymath.com - Nice step-by-step solutions. Math-centric.

http://www.symbolab.com - Really good for step-by-step solutions to math problems. Cymath alternative.

https://www.desmos.com - Amazing for plots.

https://www.mathway.com - Cool math solver and chemistry solver. Plotting somewhat similar to Desmos.

https://www.omnicalculator.com - Cool, straightforward calculator for quick results.