During gameplay press the '~' (tilde) key to bring up the cheat console and then enter the following codes to enable the effect you desire.

Toggle God Mode On/Off:
Enter 'god'.

Invisible to Enemies:
Enter 'notarget'.

Unlock ALL Guns and Extra Ammo:
Enter 'Impulse 101'.

Remove Weapon Model from the Screen:
Enter 'impulse 200'.

Toggle Flying and Moving through Walls On/Off:
Enter 'noclip'.

Wireframe Objects Only Effected by Physics:
Enter 'vcollide_wireframe 1'.

Kill ALL NPC’s in the Area:
Enter 'npc_kill'.

Spawn Scout Car:
Enter 'ch_createjeep'.

Spawn Airboat:
Enter 'impulse 83'.

Spawn Airboat:
Enter 'ch_createairboat'.

Turn On Bump Mapping:
Enter 'mat_fastnobump 1'.

Turn Off Bump Mapping:
Enter 'mat_fastnobump 0'.

Unlock Wire Frame Models:
Enter 'mat_wireframe1'.

Draw Game Triggers:
Enter 'showtriggers_toggle'.

Draw Normal Maps:
Enter 'mat_normalmaps 1'.

Draw Surface Materials:
Enter 'mat_normals 1'.