Simple idea, beautifully realised: http://www.fubiz.net/2011/03/30/dear-blank-please-blank/

Example:

‘Dear Fork,

I understand that we haven’t spoken since I ran away with dish, but I thought you should know that you have a son. His name is spork.  He has your hair.

Sincerely,

Spoon’

Of course, I also like this one:

‘Dear Mary,

Just admit that you slept with someone else.  This is getting out of hand.

Sincerely,

Joseph’

More on the backstory of that one (just in case you weren’t sure where to look) at Luke 1.26-38