If you're in need of ammunition for your currently-equipped weapon during any Half-Life 2 episode, insert the following code to max out your ammunition: Y, B, A, X, Right Bumper, Y, X, A, B, Right Bumper.
Cheat: Portal Cheats
Insert the following button commands during gameplay for the desired result...
Create Box
Down, B, A, B, Y, Down, B, A, B, Y
Fire Energy Ball
Up, Y, Y, X, X, A, A, B, B, Up
Fire Rocket Projectile
Up, Y, Y, B, B, A, A, X, X, Up
Portalgun ID 0
Up, Left, Down, Right, Up, Left, Down, Right, Y, Y
Portalgun ID 1
Up, Left, Down, Right, Up, Left, Down, Right, X, X
Portalgun ID 2
Up, Left, Down, Right, Up, Left, Down, Right, A, A
Portalgun ID 3
Up, Left, Down, Right, Up, Left, Down, Right, B, B
Portal Placement Anywhere
Y, A, B, A, B, Y, Y, A, Left, Right
Upgrade Portalgun
X, B, Left Bumper, Right Bumper, Left, Right, Left Bumper, Right Bumper, Left Trigger, Right Trigger
Cheat: Free Health
The code scheme popularly known as the Konami Code works during Half-Life 2 Episodes, oddly enough, and will generate a Health Pack for your use. The code can be done over and over again: Up, Up, Down, Down, Left, Right, Left, Right, B, A.
Achievement Hint: Dancing Vortigaunt
You can find the "dancing" vortigaunt in Water Hazard at the part after you take down the chopper. Once you open the gate, look to the opposite side and you will see a pipe. Enter the pipe (which is difficult; try using something to stand on) and follow the passage until you see a friendly Vortigaunt who talks to you.
Achievement Hint: Garden Gnome (Episode 2)
The gnome is in the room in the first chapter where Alyx tries to contact her dad. The gnome should be under the bed on the right hand side of the door you exit through. Don't forget to shut the hatch once you put him in the cannon or the achievement will not register.
Secret Achievement: Defiant
One of the secret achievements is at the beginning of Half Life 2 when the Combine guard asks you to throw the can in the garbage, make sure to throw the can at the guard first.
Secret Achievement: Where Cubbage Dare Not Tread
On Highway 17, when you reach Colonel Cubbage and have to destroy the gunship, do so from the top floor of the house. I took the gunship out from up there and got the achievement.
Easter Egg: ApertureScience.com
The various grafitti in Portal can help you crack the code for the website www.aperturescience.com. Go to the website (it uses Shockwave Flash) and type in the command LOGIN.
The user name is CJOHNSON and the password is TIER3. Once you are in, you may type in HELP for a list of valid commands. Have fun!