Mathway is a website where you can type in problems ranging from basic math to Calculus and not only get the answer, but a step-by-step explanation. All of the key vocabulary words in each explanation are linked to an online glossary. You can graph answers on the coordinate plane when needed as well.
This should be on every students' list of sites to use for homework help, as well as a great self-directed way for students to check their own work. You could do anything from a set of problems, project or test and then have students correct everything themselves and get detailed explanations all from one website. It's fantastic.
You might check out the rest of Mr. D's blog as well.