I found that thing I was looking for a few months ago, the place where people can leave behind letters for when they die.

Actually, I didn't find it through any rational process. What happened was, that I was listening to the Juliet Letters by Elvis Costello, and there's a song on it called "Dead Letter." And when I read that, I realised that the only name that that site could possibly have was something to do with Dead Letter. And it did. It's called the Dead Letter Office. Sometimes you can't beat intuition.