A farewell letter to co workers is a great way to make your leaving a job more personalized. You can mention where you are going, but don't mention anything about why you are leaving if it has anything to do with the company they are still employed with. You can thank your coworkers for things that they have done for you, or taught you. Finally, you can include your phone number, or/and address
. should you wish to contact them in the future, or if you would allow them to use you for a reference in the future. Some samples are available at this site: http://www.letterrep.com/showcatletters.php?catID=60 .