I'm not going to waste your time saying anything about the regular use of search engines, except to mention that these are my personal favorites:
Instead, let me briefly describe a couple of ways in which search engines can be of help to the English teacher.
Have you ever suspected your students of cheating by copying and pasting from articles they found on the internet? You can check if they have. Let's assume that one of your students whose skills were normally not very impressive, turned in a paper containing this passage about artist Damien Hirst: "Some call Hirst a media-junkie, but he makes extremely good copy and the analysts have repaid him by making him rich and famous. Of course it doesn't hurt that his works were bought early on by Charles Saatchi, a sure way to gain favour in all the right circles".Not very likely he wrote that himself, right? Type the whole passage into the search line of AltaVista and click "Search". I first tried with all of it within quotation marks, but that was too demanding a query and yielded no results. Then I tried without any of the symbols that AltaVista allows in its query syntax, but ended up with too many hits. Finally, I tried the plus sign + in front of the key words Hirst and media-junkie and ended up with one hit - the very article from which the quotation had been taken. The good old adage "Read the f***ing manual" goes for search engines too: the better you know the syntax of each of them, the more effective your searches will be.
Did you ever wonder if you can now actually say "take decisions" instead of what you probably learned, viz. "make decisions"? One way to find out - if you believe that you should go by usage rather than prescriptive grammar - would be to search for occurrencies of either in a concordancer. The problem of using a search engine like AltaVista or HotBot, of course, is that it would include usage by someone like myself, who is not a native speaker of English. Fortunately, we also have more specialized tools:
The Cobuild Direct Corpus Sampler contains about 45 million words from books, newspapers, transcribed speech, etc., and you can search for example for the concordance take+a+decision. My search yielded 13 occurrences. This is a demo version that only yields a maximum of 40 occurrencies.