This letter thanks someone for helping you out with work.
1234, Main Street
Boston, MA 02123
07 April, 2005
<Recipient Address Goes Here>
I really can't thank you enough for your help with <project>. As always, you went above and beyond the call of duty in coming to my aid. I don't know what I would have done without you and your kind assistance.
Now that that project is thankfully behind me, I hope you'll let me know if I can ever return the favor. Thanks again for your help!
encl: <List of enclosed items goes here>