Left 4 Dead

Left 4 Dead (shortened L4D) is a first-person survival game series created by Valve. There have been several Left 4 Dead easter eggs in the Dead Rising games, as well as to Dead Rising in the Left 4 Dead games.

Frank West's note to Otis
Probably one of the best known easter eggs in Left 4 Dead, In the Left 4 Dead 2 DLC: "The Passing", there is graffiti on the wall of one of the safehouses that reads: "Otis, No helicopter, zombies are to fast, not going to make it - Frank West". It appears that Frank West was killed sometime after he wrote this note. This could also refer to a similar note found at the end of Dead Rising.

Chuck Greene in "The Sacrifice"
Chuck Greene appears in a picture above a jukebox in the Left 4 Dead comic: "The Sacrifice".

Willamette "No Mercy" campaign easter egg
In the Left 4 Dead campaign: "No Mercy", the number 53,595 appears on the wall of a hospital. This most likely refers to the population of Willamette, Colorado, despite it being one number less than it's actual population.

Ellis wearing Case Zero shirt
On the day of Dead Rising 2's release, the Left 4 Dead 2 team sent out a picture of Ellis, one of the four playable characters in Left 4 Dead 2, wearing Chuck Greene's long-sleeved t-shirt from Dead Rising 2: Case Zero, congratulating the Dead Rising 2 team on their release.

Dead Rising 2: Case West Witch Easter Egg
Likely to be the most notable easter egg in any of the Dead Rising games, there are words written in blood in the Underground Tunnels that read: "Don't startle the wi...". This refers to The Witch in Left 4 Dead, in which the player mustn't startle The Witch, who has her hands covering her face; crying, shall the player fail to do this The Witch will react by screaming and running towards the player, who will be immediately incapacitated by her.