Cheat menu cheats
Alien Mode Up, Triangle, Up, Triangle, Up, Triangle, Up and Triangle
Infinite Ammo Down, X, Left, O, Down, X, Left, O
Invincibility Left, Square, Up, Triangle, Right, Circle, Down, X
Invisibility Left, Square, Right, Circle, Left, Square, Right, Circle
Slow motion Right, Square, Up, Triangle, Right, Square, Up, Triangle
Xen gravity Up, Triangle, Down, X, Up, Triangle, Down, X
Unlock all Decay missions
Go to the Options, then Cheats. To enter the code you must have two controllers.
All Decay missions unlocked including bonus mission Xen Attacks
p2_R3, p1_L3, p1_up, p1_up, p2_down, p2_down, p1_left, p1_right, p2_left, p2_right, p1_X, p1_O, START
Bonus Alien Mission
Get an A-grade on all 9 of the Decay missions. If you successfully do this, you'll get to play a 10th level as a Vortigaunt alien. Getting an A-grade on this level will formally unlock the Alien Mode. (Note: the best way to ensure an A-grade for a level is to get at least 25 kills.)
For this code to work, you will need a copy of Playstation Magazine Issue 57 (The one with Jango Fett on the cover) and it's demo disc. From the Half-Life main menu go to option and the the cheats menu. Enter the following cheat: Triangle, Square, Left, Right, Circle, X, Up, Down. It should open an extras menu on the options screen. Go to the extras menu and it should say opening disc tray. When it opens remove the Half-Life disc and put in the issue 57 disc and it should load the Half-Life Uplink Mod. Enjoy!