MW2:

To enable codes type them while holding down CTRL-ALT-SHIFT

blorb = Toggle invulnerability
cia = Toggle unlimited ammo
coldmiser = Toggle heat tracking
hangaround = Toggle mission time-limit
flygirl = Add jumpjets to mech
mightymouse = Toggle infinite Jumpjet juice
gankem = Destroy targeted mech
enolagay = Explode an atom bomb (killing everything?)
michelin = Toggle bounding spheres on debris and mech parts

tinkerbell = View battle from free-floating camera. Turn off with "c".
ctrl-left to turn left
ctrl-right to turn right
ctrl-up to increase altitude
ctrl-down to decrease altitude
z to move forward
shift-z to move backwards

front = Toggles mini-front-view, shown in place of mini-rear-view
xray = Enhanced imaging with see-through. Turn off with "w".
dorcs = view the MW2 Sim Programmer Dorcs Web pages
icanthackit = End mission successfully
idkfa = End mission as a failure
zmak = Toggles Time expansion (?)
meepmeep = Time compression key enabled (?)
unmeepmeep = Time compression key disabled (?)
lairdo = "ATTENTION ENEMIES: Don't mess with the blimp." (?)
dei = "F E I R". (?)
sh*t = "Freebirth vulgarity will not be tolerated!"
fu*k = "Freebirth vulgarity will not be tolerated!"

These codes work in the Trials of Grievance instant-action missions, by
changing your name in Star Config to:

Calvin = Adds Elemental between Dire Wolf and Fire Moth (1 of 3)
Hobbes = Adds Tarantula between Elemental and Fire Moth (2 of 3)
Enzo = Adds Battlemaster IIC between Tarantula and Fire Moth (3 of 3)
This code works in the campaigns, by registering with the name:
FREEBIRTHTOAD = Lets you pick and play any of the 16 missions

GBL:

Hold down CTRL-ALT-SHIFT and type:

clark = X-ray vision
kent = Invincibility
dorcs = View the MW2 Sim Programmer Dorcs Web pages
kaboom = Destroy all Mechs
palex = Destroy targeted mech
putz = End mission successfully
jumbo = Add jumpjets to mech


MERCS:

Hold Control-Alt-Shift and type:

superfunkicalifragisexy = Invulnerbility 
iseenfireandiseenrain = Unlimited Ammo 
ooohhhlllaaalllaaa = Heat tracking 
itsdabooomb = Makes targeted enemy self destruct (doesn't work on
things that are not high lighted in red brackets) 
crazysexycool = Infinite jumpjets 
inmybeautifulballoon = Adds jump jets 
likethecomstarbaby = Automatically succeed in mission (it doesn't set
your mission goals as successful though) 
ontimeeverytime = Time compression key enabled. 
antijolt = Time expansion enabled 
bubbleboy = Bounding spheres 
beholdmyglory = Imfree-eye mode on
undflashyflashy = AutoGrouping enabled (makes all weapons fire from
the first trigger) 
walkthisway = Leading Recticle enabled (makes enemies follow your 
targeting crosshairs towards you) 
wediditagain = Same as dorcs in MW2 
doredjackandtikrules = ???