Get the indicated number of consecutive kills in Multi-player mode to unlock the corresponding bonus.

UAV: 3 (reveals enemies on mini-map)
Care Package: 4 (ammo or killstreak air drop that can be collected by anyone)
Counter UAV: 4 (disables enemy maps)
Sentry Gun: 5 (airdropped weapon)
Predator: 5 (directed missile)
Precision Airstrike: 6 (select a location to bomb)
Harrier Strike: 7 (select a location to bomb and defend with a Harrier)
Attack Helicopter: 7 (helicopter backup)
Emergency Airdrop: 8 (four ammo or killstreak air drops that can be collected by anyone)
Pave Low: 9 (armored helicopter backup)
Chopper Gunner: 11 (operate a helicopter gun)
AC130: 11 (operate a plane gun)
EMP: 15 (disables enemy electronics)
Tactical Nuke: 25 (kills everyone)

Die the indicated number of times in a row without a kill in Multi-player mode to unlock the corresponding bonus.

Martydom: 4 (drop a live grenade just after dying)
Painkiller: 3 (health boost when you spawn)
Copycat: 4 (steal your killer's class in killcam)
Final Stand: 3 (get back up after being wounded)