This thread could contain spoilers, so be wary when reading if you don't want anything ruined
I've found two Easter Eggs in HL2 so far:
1. Dr. Kleiner's pet headcrab is named Heady Lamar. Headly Lamar was the villain in the movie Blazing Saddles, one of the jokes in the movie is that he is constantly being called "Heady" instead of "Headly."
2. Star Wars: Empire Strikes Back reference. At one point while you've got a squad following you inside a building you come across a hole in the floor; one of your squad members comments "yeah, that'll do nicely" which is what Han Solo says about the cave in the asteroid field in ESB.
Anybody find any other Easter Eggs?