You've probably at least heard of Alone in the Dark, arguably the grandfather of survival horror games. Hell, you may have even seen my previous one-shot Let's Play of it on Halloween 2012. So what is Jack in the Dark?
In a nutshell, Jack in the Dark was a standalone demo for Alone in the Dark 2, included as a freebie with the CD version of the first game. Its main purpose was to showcase the improvements in the engine, from the simplified interface to the fact that the run command actually works better now (no more tricky timing on where and when you have to double-tap the arrow key).
While Jack in the Dark is very short and unrelated to the rest of the series plotwise, it does have a few in-jokes that tie in to the second game. Grace Saunders is the secondary protagonist in Alone in the Dark 2, and the villainous jack-in-the-box in this game strongly resembles that game's villain, One-Eyed Jack.