Choose a platform:

Playstation2

PC

Xbox

During gameplay, pause the game, then enter the following codes - using the D-pad for directions - and hit [start] to activate the cheat.

Replensihes Health Meter, TK Meter, and Reaver Gauge:
Left, Right, Left, Right, R1, L1, Circle, Triangle, Down

Gain All Health Fragments, TK Runes, and Tomes:
Left, Left, Up, Up, L1, R2, Circle, Down, Triangle
(This gives maximum health and Telekinesis meters)

Invinciblity:
Up, Down, Right, Down, R1, R2, Down, Triangle, L1

Infinite Reaver Charge:
Down, Down, Up, Left, R1, R2, Down, Triangle, Circle

Unlock All Slams/Special Attacks:
Right, Down, Up, Down, Down, R1, Triangle, Circle, Down

Enables Wire-Frame mode:
L1, Down, L1, Up, R1, L2, L1, Down, Triangle

Disables Textures:
L1, Down, R2, Right, R2, Up, Triangle, L1, Down

Unlock All Dark Chronicle Entries:
R1, Down, R2, L1, Right, L2, Triangle, Down, L1

Unlock All Bonuses Material:
R2, Down, L2, R1, Left, L2, Down, L1, Triangle

Enable Tube Reaver (Works for Kain only):
Up, Down, Left, Right, R2, L2, Triangle, Down, Circle

Toon Graphics:
Up, Down, Up, Down, R1, R2, Down, Circle, Triangle
(WTF were they thinking...?  >_< )

Many more cheats and hacks implementing cheat devices for the PS2 can be found at The Lost Worlds.

Enter these cheats on the Pause menu screen.  Before entering them, be sure to set the control type to 'Keyboard' in Control Options, otherwise they won't work. 
Note: The North American release of the PC version needs to have the Unofficial v1.1 patch applied to it in order to make these codes work.  The European PC version does not need this patch.  To see a list of what else this patch fixes, visit The Lost Worlds' Defiance Patch page.

Replensihes Health Meter, TK Meter, and Reaver Gauge:
Left, Right, Left, Right, Feed, Autoface, Telekinesis, Lift Attack, Down

Gain All Health Fragments, TK Runes, and Tomes:
Left, Left, Up, Up, Autoface, TK Aim, Telekinesis, Down, Lift Attack
(This gives maximum health and Telekinesis meters)

Invinciblity:
Up, Down, Right, Down, Feed, TK Aim, Down, Lift Attack, Autoface

Infinite Reaver Charge:
Down, Down, Up, Left, Feed, TK Aim, Down, Lift Attack, Telekinesis

Unlock All Slams/Special Attacks:
Right, Down, Up, Down, Down, Feed, Lift Attack, Telekinesis, Down

EnablesWire-Frame mode:
Autoface, Down, Autoface, Up, Feed, Autoface, Down, Lift Attack

Disables Textures:
Autoface, Down, TK Aim, Right, TK Aim, Up, Lift Attack, Autoface, Down

Enable Tube Reaver (Works for Kain only):
Up, Down, Left, Right, TK Aim, Lift Attack, Down, Telekinesis

Toon Graphics:
Up, Down, Up, Down, Feed, TK Aim, Down, Telekinesis, Lift Attack
(WTF were they thinking...?  >_< )
 

For the following cheats, I've found that in addition to entering them in the same fashion as the above codes, you'll need to save a game and go back to the title screen, in order to see the appropriate effect(s):

Unlock All Dark Chronicle Entries:
Feed, Down, TK Aim, Autoface, Right, TK Aim, Lift Attack, Down, Autoface

Unlock All Bonuses Material:
Down, Telekinesis, Up, Left, Right, Right, Lift Attack, Down, Right, Right

During gameplay, pause the game, then enter the following codes - using the D-pad for directions - and hit [start] to activate the cheat.

Replensihes Health Meter, TK Meter, and Reaver Gauge:
Left, Right, Left, Right, R, L, B, Y, Down

Gain All Health Fragments, TK Runes, and Tomes:
Left, Left, Up, Up, L, Black, B, Down, Y
(This gives maximum health and Telekinesis meters)

Invinciblity:
Up, Down, Right, Down, R, Black, Down, Y, L

Infinite Reaver Charge:
Down, Down, Up, Left, R, Black, Down, Y, B

Unlock All Slams/Special Attacks:
Right, Down, Up, Down, Down, R, Y, B, Down

Disables Textures:
L, Down, Black, Right, Black, Up, Y, L, Down

Unlock All Dark Chronicle Entries:
R, Down, Black, L, Right, Black Y, Down, L

Unlock All Bonuses Material:
Black, Down, White, R, Left, White, Down, L, Y

Enable Tube Reaver (Works for Kain only):
Up, Down, Left, Right, Black, White, Y, Down, B

Toon Graphics:
Up, Down, Up, Down, R, Black, Down, B, Y
(WTF were they thinking...?  >_< )