Agility:
When your adrenaline reaches 100, press Down, Down, Down, Up on the D-Pad.

Berserk:
When your adrenaline reaches 100, press Up, Up, Up, Up on the D-Pad.

Regeneration:
When your adrenaline reaches 100, press Down, Down, Down, Down on the D-Pad.

Invisible:
When your adrenaline reaches 100, press Right, Right, Left, Left on the D-Pad.

Unlimited T.A.G. rifle:
Are those lousy clan members keeping this from you? Well, here it is! Pick up the T.A.G. rifle and hold fire while keeping still on your target. The moment the laser turns red, hold your previous weapon and next weapon buttons at the same time until the laser turns an even brighter red, then fires. As soon as the weapon hits, you may then let go of both weapon selection buttons, then let go of your fire button. Your ammo may show 0, but it's unlimited unless you select another weapon or die. Warning: Using this on Xbox Live! might get you banned!

Thanks to Revolution readers Gareth John, Gavin Alford, Ender and TempesT!