Mathway is a Web calculator that not only solves math problems for you, but also shows you how it got to the answer with step-by-step directions. It's the kind of service that would have utterly ruined me in middle school if I had wanted to cruise through the stacks of homework without doing any of the actual computations.
Mathway covers several types of math genres, including high school level stuff like trigonometry and calculus. It'll also take any "basic math," like what you'd do with a calculator, although it's kind of a waste since most problems only involve one line of explanation. I'm guessing most people would simply open up their computers' calculator instead.
In addition to its problem solver, Mathway has a built-in graphing tool and a glossary--just in case the solver throws some terminology around that you haven't heard before. You can also embed any answer to show it off to others--although I'm betting more people are likely to use the e-mail link instead.