Dear Mr. (Mrs.)..........

Despite the fact that I have enjoyed working for you and your organization 
immensely, I would like this letter to serve as a two-weeks notice and a 
letter of my resignation. 

Working for you was challenging and intellectually satisfying, but the 
opportunity presented to me at this time is rare and would be a mistake to 
pass on.

Thank you for all you have done for me.

Sincerely,