Completion in GTA IV

Required For 100%

Complete the main story missions
Complete all 10 drug deliveries for Jacob Hughes
Win all 9 races for Brucie Kibbutz at least once
Exotic Exports for Brucie
Complete all 9 Assassination missions
Complete all 30 car thefts for Stevie
Brucie - Unlock special ability
Jacob - Unlock special ability
Packie - Unlock special ability
Brucie - Do each available activity once
Jacob - Do each available activity once
Packie - Do each available activity once
Beat a computer or friend at Pool
Beat a computer or friend at Darts
Beat a computer or friend in a full game of Bowling
Get the high score in the QUB3D arcade game
Do all random character missions (excluding Jeff, Ivan, Clarence, and Cherise Glover)
Complete all the Most Wanted missions via police computer
Do 20 vigilante missions via police computer using the "Recent Crimes" option
Kill all 200 Pigeons
Complete all 50 Unique Stunt Jumps

Once the game is completed 100%, you can buy an unlimited amount of ammunition.
Killing all 200 pigeons will unlock the Annihilator helicopter on an Alderney helipad.
When completing all 10 drug delivery missions, Little Jacob will tell you about a Rastah Huntley behind the Café (one time only).