#1: In Dead Air, before the finale, while still in the safe room, sometimes you can see a box which advertises Valve Software's "Team Fortress 2".(Mostly works on Xbox version)
#2:The APC in Blood Harvest that says "TK-421" is a reference to a stormtroopers in Star Wars that Luke Skywalker and Han solo incapacitated.
#3: At the Boathouse in Deathtoll, even on expert, a healthpack and pills spawn in the bathroom across the field from the house.
#4: You can see th burning city of Newburg from Dead Air at the end of Death Toll
#5: Tanks float when dead.
#6: The open area at the end of Death Toll is a great area to get the "Man Vs. Tank" Achievement. There is enough room to fend off the hoard yourself and enough to deal with the tank.
#7: John Slater, who voices the person who drives the boat in Death toll also voices the Sniper in Team Fortress 2.
#8: In the hospital levels of No Mercy, there are alot of X-rays with only 4 fingers, referencing to the cover art.
#9: On the movie poster for No mercy, Zoey has 3 hands holding her gun.(You have to look very closely.)
#10: If you are a Infected when the elevator in No mercy is activated and heading down, the number of what floor it is on will not change from "04" in reference to the title.
#11: In the hospital chapters starting safe room, there is graffiti on the wall saying "I killed..." and then listing different variables, including 53,594, the number of infected you have to kill to receive the "Zombie Genocide" Achievement.
#12: At the beginning of the sewers in No Mercy, if any gas tank catches fire, or shot, the entire station explodes into flames.
#13: Near a escalator in The subway, there is a ad for "Orange Box" Juice, referencing to one of Valves vastly popular game packages.
#14: In some of the newspapers that you can see throughout the game, Dr.Breen can be seen from "Half Life".