What were the effects of Norman Conquest on the development of the English language?

You;ve gotten several excellent answers here so far, and I have very little to add that might be helpful.  There are plenty of more sophisticated way to talk about this historical event and its ramifications.  My students have always found this interesting, though, so I share it with you, as well.

The French-speaking Normans, after the conquest, became the overlords of the Old English-speaking Anglo-Saxons. Language became one indicator of class and social status; speaking French was a signal to others that one had a certain social standing (which is why Geoffrey Chaucer, who wrote The Canterbury Tales in English in order to be accessible to the common man, was villified by his wealthier and higher-class peers.) 

One interesting carryover still in our language is the words for animals and meat.  The Anglo-Saxon serfs were the tenders of animals meant for the tables of their rich overlords; they rarely saw any of the meat from those animals.  The French who lived in the manors rarely saw the live animals but certainly ate the meat they provided.  That's why we call the animal "swine" (Old English) but the meat from a pig is called "pork" (French).  The same is true for cow and beef, sheep and mutton, hen and poultry...and the list goes on. This principle is undoubtedly true for other areas of our language, but this is the one with which I'm most familiar.

Eventually, the Plague was the great equalizer of both class and language in Britain.  When the French landholders were forced by necessity to grant land to the English, the distinct divisions in language became rather a co-mingling--the foundation for the Germanic languages we have today.

Hope you find that helpful--or at least interesting!