There was a time when a previous employer looked like we were going to go down in flames -- 2008. I wrote such a love letter in the main include file (yay PHP) that told them how to figure out how the application worked and gave a credit blurb to all the previous devs and how they helped build the application.

We didn't go under quite yet and it was my extreme pleasure to allow two more devs to write their own blurbs and edit the letter to help future others. The company later went under and was acquired by a competitor, so I'm sure they've seen the letter in order to figure out how to extract data from the system. Effort not wasted.