These are a few Easter eggs for Left 4 Dead 2.
1: The tattoo on francis's arm is the same as the ring on Nick's finger.
2: The gnome "Gnome Chompski" in "Dark Carnival" is named after the famous philosopher Avram Noam Chompsky.
3: In the safe room where you collect diesel on "Hard Rain" you see remarks from Erik Faliszek and Erik Wolpaw who are writers of Valve.
4: In "The Parish" the jukebox plays song by Jonathan Coulton like "Still Alive" and "Your Brainz"
5: In "The Passing" Frank West from Dead Rising wrote on the wall in the bar next to the pool hall.
6: In "The Passing" right when you get out of the pool hall is shows the Midnight Riders tour bus drive away behind the fence.
If there is anymore you know of please comment and tell me.