Inside the garage is a purple ball hidden in the rafters. Knocking it down and landing it through the hoop on the back of Sam's room's door will trigger a secret journal entry from "Mitten", and play a sequence of cat images on Sam's TV.
Contributed by EFB01
A salad dressing bottle can be found hidden in the kitchen pantry behind some boxes. Upon inspection, the label bears a striking resemblance to the cover artwork for BioShock Infinite. The brand name, "Levine's", is a reference to BioShock's Creative Director Ken Levine, who use to work alongside the founding members of the Fullbright Company at 2K Games before the development of Gone Home.
Contributed by DJN
Opening the safe in the basement, Spoiler:using the code 1963 (the year Terry was last measured on the wall), will reveal that Oscar Spoiler:likely committed suicide after he attempted to reconnect with his sister, who returned his letter back to sender, meaning that she likely never read it. The letter reveals that Oscar intended to commit suicide if he couldn't have his sister back in his life. Besides this letter in the safe are a number of different poisons and drugs.
Contributed by Dazz
The Fullbright Company built a prototype of the game in Frictional Games' HPL2 engine, and asked if they could use the engine in the final game. They were turned down as a matter of policy, and encouraged to use the Unity engine instead, which the game was eventually completed in.

After playing the final version, and being informed of its history, Thomas Grip of Frictional Games later asked to see the prototype, stating that he had "Mail Steve about Amnesia: Gone Home" written on his todo list for six months.
Contributed by Mass Distraction