In the first St. Petersburg mission, there is a building called "Zaitsev Building". It was obviously named after the famous soviet sniper Vasily Zaitsev (immortalised in the movie "Enemy at the Gates“). Quite fitting, if you think about progress of the misson ...
On the mission "Shogun Showdown", 47 enters a room with a lot of monitors and a big red button to press. In this same room, there is a bookcase. Looking on the titles of the books, you can find one titled "Vamos Arregaçar". This is Brazilian slang for something like "Let's kick some ass!". It looks like they had a Brazilian guy in the development team...
At the very beginning of the "Graveyard Shift" mission, you can see a video camera pointed at a coffee pot. The camera plays a part in one of the in-game puzzles in that mission.
What you might be wondering is why anyone would dedicate a video camera just to watch over a pot of coffee. It's possible the coffee pot camera is a homage to this:
The game's main villain, Russian arms dealer Sergei Zavorotko, bears a remarkable resemblance to a character in Arnold Schwarzenegger's Eraser movie that introduced us to the joys of railguns.
That character, also named Sergei, was also a Russian arms dealer and a secondary villain who showed up at the end of the film. Both he and the Sergei from Hitman have surprisingly similar appearances. Both have shoulder length hair, a thick moustache, and both wear wool turtlenecks under a red leather coat.
The Sergei from Hitman is balding on top, while the Sergei from Eraser isn't, but otherwise the two characters look almost exactly the same. It's possible the character design for the Hitman Sergei was inspired by the character from the Eraser movie.
In the "Jacuzzi Job" mission check out what's on top the small counter in the bedroom... That's right! It's a Dopefish spring-toy!
And for further Id related sightings check out the monitors in the previous "Graveyard Shift" mission: most of them are playing Quake Arena. Even the sysadm!
In the Shogun Showdown mission, there's a big security workstation on the castle's 3rd floor that monitors the entire stage. On its main screen however, you can see a screenshot of IO's upcoming (at the time anyway) Freedom Fighters! Though if you look carefully (use 1st person view = F1) you can see a few changes in the interface, such as a radar screen added next to the health bars.
On the Hitman 2 CD, you will find some goodies: The hitman 3D Studio Max model, some early screenshots of the game, which are quite humourus, and some wallpapers.
Do you know the cool tv-ads for Hitman? They are made by the Hitman-developers, not by an agency...