Easter Eggs are hidden references to other games or pop culture.
Natural Selection 2
The Natural Selection 2 Poster can be found inside the Wreck in the Grassy Plateaus and in the Aurora, and can be placed inside your Seabase. There are many other objects from NS2 in Subnautica as well. Trello is a reference to another game made by Unknown Worlds.
The first message broadcasted from the Radio, which used to say: "This is an automated bounce back from the Aurora Mining Vessel. Your distress signal has been received. An emergency relief team will be dispatched to your location. *Static* Estimated arrival time: 9...9...9...9...9 hours". The wait time is a possible reference to Portal 2, whereas the announcer says to Chell (the protagonist): "You have been in suspension for nine...nine...nine...nine...nine..."
A fifth message is received by the player's Radio later in the game. The day this message is received varies. It displays: "You've been down too long in the midnight sea. Oh don't you see what I mean?" This message is a reference to the song Holy Diver by the US heavy metal band DIO.
When the Player fabricates the Survival Knife for the first time, the Dialogue says: "Weapons were removed from standard survival blueprints following the massacre on Obraxis Prime. As an essential survival tool for 2.5 million years, the knife remains the only exception. Sorry about that." Obraxis is a developer of Subnautica.
There is a Data download mentioning the hive mind of Strader VI. Cory Strader is a developer of Subnautica.
In the PDA dialogue for the Stasis Rifle as well as in one of Paul Torgal's journal entries, a creature called a "Starwhal" is briefly mentioned. This could be a possible reference to the game Starwhal where players must control flying narwhals with the goal to pierce each other's hearts with their horns.
There is a chance for the Cyclops AI to say: "You are the best captain on the planet, I'm not even squidding." when the Player has entered it, rather than its normal line. This refers to the Player being the only captain on the planet.
The Legend of Zelda
Found in Alien Relic 7.