I recommend Wolfram Alpha to all of my math and physics students, and to many others. It calls itself a Computational Knowledge Engine which doesn't do too good a job of describing itself but it is very useful as i'll explain below. It does quite a number of things that aren't comparable to other search engines.
First, one of its central components is based on Mathematica which is a mathematical programming language. Because of this it can solve problems in algebra, geometry, calculus, statistics, matrices and many other subjects. This is largely what I use it for; as in if I want to quickly solve or check a problem. If i can't remember exactly what the half angle integration of tangent is, or if a problem results in an answer to large for my calculator to display.
Second, it has large data sets available to it. These vary from current and historical weather data, i.e. what is the current temperature/chance of rain and what was the temperature june 4th 1966. The data sets also include information on size and time, i.e. what is the radius of the earth, distance to the sun/moon or how many days until the end of the year and so on. These data sets cover a huge range of topics and so many questions can be answered easily.
It also includes a dictionary, thesaurus, related topic links and basic information about anything topic you search.
To conclude Wolfram Alpha is a powerful search engine that covers a wide range of practical topics centered around science, problem solving and learning which could help anyone using this site, tutor or tutee.