Lectori Salutem said:
One character mentioning the book 'Roadside Picknick' in Metro 2033.
For those who don't know, it has similar elements as the book Metro 2033 and is also the book the Stalker games are heavily inspired by.
And it's one helluva good book, too. I really liked that touch, too.
Another thing of notice in Metro: when you're in the old command bunker, there is a boot in a corner with a small plant growing in it, as a bit of a shoutout to Wall-E.
Now, while it was -very- unhelpfull, only really a bunch of textboxes, I did like the in-game tutorial of Hearts of Iron III, where a certain short, ugly ol' Austrian with a mustache give you the basic ropes of world conquest (with the odd, 30-minute raving tantrum), before dejectidly resigning to shoot himself in his bunker once he grasps that you still don't know what on earth you're doing.