This letter is to express my gratitude for all the benefits (the company) has brought to my life. I have gained experience in the business of (whatever the company makes or sells), I have learned how to perform the job well, and I've made many friends with whom I hope to keep in contact.
With all this, it has been a hard decision for me to accept a new post, in a different line of business. But there come times in everyone's life when a change is for the better: This is one of those times. Therefore, please accept this as my resignation, effective July 15.
I will of course leave my current projects as complete as I can.