[NOTE: Minor spoilers related to the story of Half-Life 2 follow.]
I can't help but notice certain similarities between the story of Half-Life 2 and the wonderful "Tripod Trilogy" written in the late 1960s by John Christopher (pseudonym of Samuel Youd):
1. In the trilogy, humanity has been enslaved by conquering aliens, but there is a resistance movement. In Half-Life 2, humanity has been enslaved by conquering aliens, but there is a resistance movement.
2. In the trilogy, the landscape is dominated by menacing three-legged "tripods." In Half-Life 2, one of the most dangerous enemies is the three-legged "strider." (Of course both of these may simply be referencing the tripods from H.G. Wells's novel The War of the Worlds.)
3. In the trilogy, the first book is called The White Mountains, which is the location of the main human resistance organization the heroes are trying to reach. In Episode 2, Alyx points out White Mountain to Gordon, the landmark for the human resistance base (the White Forest Rocket Facility) that they're trying to reach.
Coincidence? Or a nice homage?