The good old stuff of the murderer trade, exclusive to Dark Brotherhood members that either will shell out a lot of coin or be far enough along to nab the ones in Fort Farragut.
Except no!
Recently, I finally realized that even non-Dark Brotherhood members can get into the otherwise-locked Fort by using the secret entrance in the tree stump nearby. No one will even be there.
So the 10 -free- Poison Apples that respawn every three days are there for anyone's taking.

On that note, I don't use them as often as I could. It's a little difficult to remove all the food from an area and/or reverse-pickpocket them into the intended victim's pocket.
Is there anyone that actually utilizes them frequently?