If you type the point
200 -4096 1154 in the Go To Coordinates dialog, the following dialog appears:
The voice command
voice_wtf makes the player say
Whiskey Tango Foxtrot!
Half-Life: Blue Shift
- Chapter 2: Insecurity
- Get the gun and all the ammo at the shooting range (17/51 in total) and go to the locker room. Open the locker to the left of where the armour hangs - the one that says "Calhoun" - and you will discover a box. Shoot up all your ammo into it, and it will break, revealing a chumtoad (purple alien frog).
- In the locker room, find the toilet that is out of order, climb the toilet seat of the nearby toilet and jump to peek at the back of the closed door, and you'll find a poster of a face.
- At the monitoring stations, if you watch the monitor showing the two scientists long enough, they will meet up and one of them will then do a little dance.
- Chapter 4: Captive Freight
After having taken the hidden elevator down to the old lab, climb atop of the broken duct and look into the intact part to discover a face.
- Chapter 5: Focal Point
While in Xen, find a giant pool, turn on God mode, dive in, swim all the way to the bottom, find a wall with a crack in it, break it, and you'll find the entrance to Chumtoad's Lair. You can also find snarks in this area.
You can find Chumtoads lair without god or noclip, locate the body of liquid suggested in the previous paragraph(I recommend that you have at least above 70-80 health.) Now dive in the deep pond, go to the very bottom and look for a crack on a wall, make sure you don't run out of health, if your health gets below50 or 40 before the wall is destroyed, quickly rise to the surface for air, (repeat until you find the crack in the wall,) then as soon as you find the crack, quickly break the wall the crack is on and go through the pipe then break the plant web, and jump out then you find Chumtoad's lair, In there you find three chumtoads then they disappear, and you find 10 snarks for use.
The above easter eggs above are demonstrated visually in these videos:
Half-Life: Opposing Force
opposing force.gcf\sound\drill\dr2_add04.wav: "It's time to kick ass and chew bubble gum, and I'm all outta bubble gum."
Win the Game
opposing force.gcf\sound\misc\dsbossit.wav (unused sound): "To win the game you must kill me, Randall Pittsford." 
GoldWave→Effects→(Reverse + Playback Rate→16000 + Filter→Noise Reduction→Initial noise)
Open the console and enter
haiku to make the game put together a random haiku for you. All the lines can be found in the file opfor.dll.
NOTE: Make sure you have a game started for the haiku to work.
Letters in "On A Rail"
Beneath a certain set of stairs, you'll find a crate. Break it, and destroy the vent cover that was underneath the crate. Crawl into it, and you will eventually reach a room with some satchels, a dead marine and a headcrab. On one of the walls to your left (relative to the way you came in) you will see some uppercase letters made out of brushes. It is a bit hard to spot at first, as it is textured the same as the walls are, but the flashlight makes it a bit easier.
Half Life 2
In the last map of the chapter you arrive in a wide open mud flat area, bordered by the dam leading to Black Mesa East, containing a combine warehouse and a attacker helicopter. For this explanation, assume that the helicopter is already taken care of.
At the start of the map, come out of the tunnel, through the mini "canyon" and turn left. Drive lengthways up the map and through the gateway until you reach a small wall at the very far end with two pipes in the side; the right one is blocked but the left one is open. Next, enable god mode or noclip (whichever is easiest to get past the radioactive sludge) and follow the pipe into the hill. At the end, you should come out into a cave where a lone Vortigaunt is roasting some headcrabs. :)
If you do not want to cheat and go with the Half-life 2 story Go to the warehouse with the metro police and grab the health, battery, ammo. After that, your health and power should be 100/100 now locate the tunnel suggested in the previous paragraph, using the airboat to prop you up, get in the tunnel then now sprint quickly on the sludge while jumping repetitively till you find a corner pipe, now quickly sprint up to the camp looking grounds, now that you health stopped dropping you can relax until you get back out,them being forgiving in this game, they supplied you with health kits and batteries make sure you take them before making a trip back outside.
Team Fortress 2
This is activated on the birthday of the original Team Fortress, which is either August 24, when the Team Fortress was released as a Quake total conversion, or December 22, when the game was released standalone on the QuakeWorld 2.0 engine. All players wear birthday hats, all gibs turn into birthday presents and any time you hit someone with a weapon, balloons slowly float away out of them.
You can find birthday servers by typing in the search box in the custom tab birthday, then it should list the servers with a birthday cvar on.
This funny wet floor sign can be found exploring "portal content.gcf" archive with GCFScape tool.