Unlock extra stuff:
     Beat  the  following  levels  under  the specified time to unlock
multiplayer levels and more skins:

Level:             Mode:          Time:          Unlock: 
Subway             Agent          2:15           Castle level 
Masquerade         Agent          3:15           Air Raid level 
Night Watch        00 Agent       2:20           Forest level 
Turncoat           Agent          3:20           Capture The Briefcase level
Whole game         00 Agent       Any            Golden Gun level 
King's Ransom      Agent          2:20           Team King Of The Hill level
Cold Reception     Secret Agent   3:15           Sky Rail level
City Of Walkways   Secret Agent   3:45           Covert Skins 
Masquerade         00 Agent       4:20           Scientist skins 
City Of Walkways   Agent          3:35           Civilian skins 
Cold Reception     00 Agent       3:25           Exotic skins 
A Sinking Feeling  00 Agent       2:55           Navy skins 
Midnight Departure Agent          3:05           Soldier skins 
King's Ransom      Secret Agent   3:45           Security skins 
Whole game         Agent          Any            Contemporary Skins 
Whole game         Secret Agent   Any            Classic skins 
Courier            Secret Agent   2:00           Suit skins 
City of Walkways 2 Agent          3:00           Wildfire mode 
Thames Chase       00 Agent       4:25           Exotic weapons mode 
Fallen Angel       Secret Agent   2:45           Gadget war mode 


Health at 100%:
     In  the  mission  where you chase the female assassin (mission 3)
Thames  Chase,  when  you  are  defusing  the  bomb, stop the clock on
exactly  0.07  seconds  left and your health will be topped back up to
100%.

Bye, Bye Helicopter:
     I found a useful strategy for destroying the attack helicopter on
City  of  Walkways  2.  First, kill the guy on the box with the Rocket
Launcher,  (00 Agent only), with the Meyer TMP, or the Wolfram P2K, so
he  doesn't  blow you up while you're doing this. Go to the car on the
lower  level,  and  pick up the Sentinal and some ammo. Then, go over,
(on  the same level), and stand in the spot where you can pick up more
Soviet ammo. Aim the Sentinal through the gap between the ramp and the
upper  floor. From here, you should be able to see the helicopter when
it  pauses its movement. Shoot it with the Sentinal during this pause.
Eventually,  the helicopter will blow up, (if you hit it). (NOTE: It's
not a good idea to use this hint if you're trying to get Wildfire Mode
in  Agent.  Usually,  the  final mission time using this hint is about
8-10 minutes.)

There's an easier way:
     On    mission   12,   Fallen   Angel,   after   you   enter   the
finger-print-locked door, don't move. Take out any gun but the Soviet,
(since  it's  not  that accurate), and shoot the two security cameras,
(one  on  the  far right wall, and one on the closer left wall). After
that, just run over to the stairs, and the doors will not close and no
gas will spray out.

Extra 9mm Ammo:
     If you happen to like the guns that use 9mm ammo, here's a way to
get  some extra. With any guard on any level, use either the Stun Bomb
or  the  Stun function on your wristwatch to stun an enemy. After they
drop their gun, wait until they pull out a Wolfram P2K. Kill them, and
you'll have some extra 9mm ammo.