Wolfram Alpha was built to provide knowledge and solve problems. The capabilities are vast, but some features are behind a paywall.
Desmos is a free online graphing utility. It also has a large selection of pre-made lessons for teachers.
GeoGebra is an online graphing calculator and also creates nice geometric diagrams.
Khan Academy creates high quality content for many different subjects. For ease of finding links to use for their videos, I would recommend looking at the playlists on their YouTube Channel.
Math Cracker provides calculators for many different algebraic problems and statistical tests.
Kuta Software is useful in creating worksheets for many different mathematical courses.