Phras.in is a free web site that can compare two phrases or two words and let you know which one is better (or correct) by taking into account the number of Google hits that each one has got.
Simply enter two phrases or words in the fields and hit the “Contextualize ‘em” button.
The number of results is shown next to them. Looking at this you can decide which one to use. If you are using a word with wrong spelling a suggestion is shown next to that field.
You can also get results directly from the address bar by typing in “http://phras.in/phrase1/phrase2” [via Download Sqad]