Contributed by Retsof-Noraa
In original copies of Half-Life, During Chapter 3, there is a secret room in map c1a1c that can only be accessed by enabling cheat mode and noclipping through the floor. The room is made of tiled pictures of Valve co-founder Gabe Newell. The room still exists in later versions of the game, although the imagery of Newell has been removed and attempting to enter the room takes you back into the normal map.
Contributed by ThisIsGamer
Gordon Freeman's original design was nicknamed Ivan the Space Biker, his beard and futuristic suit are what got him this name.
Contributed by DidYouKnowGaming
An unused enemy named "Mr. Friendly" can be found in the Half-Life's data. According to the book "Half-Life 2: Raising the Bar" Mr. Friendly's attacks included knocking Gordon's weapons out of his hand, knocking off Gordon's glasses (making the screen go blurry) and even raping the player. It was created by Ted Backman, and the design was because Ted "decided to go with something that elicits a homophobic response". The in-game texture is also signed by Ted.