This is what I just used about 4 months ago. I really liked working for this company, and was sad to go, but I needed new challenges. So that's the tone I set with the letter. If you hated your old company, write one with some reasons why you were leaving and be polite about it if you want to continue using them as a reference.
My Name
My Address
March 9, 2001
Manager Name
Company Name
Company Address
Dear Manager,
This letter is to inform you that I have been offered an opportunity to work as a software developer for a bank in New City. Therefore I will be resigning my position here at Old Company effective Friday, March 16, 2001.
This decision was not an easy one to make. I have enjoyed the time I have spent working and learning here. However after much consideration, I have made the decision to accept the new job. I would like to thank Company for giving me the opportunity to work here and for providing an excellent work environment.
I will miss the Company family and I wish all the best in the years to come.
Sincerely,
Me