here's the codes found so far:
please note, cheats 1 - 4 will block the 'Cleaned The Mean Streets' achievement while 5 and 15 will do the same for 'Walk Free'.
1, Health And Weapons - GTA-555-0100
2, Weapons - GUN-555-0150
3, Advanced Weapons - GUN-555-0100
4, Health - DOC-555-0100
5, Wanted Level Down - COP-555-0100
6, Wanted Level Up - COP-555-0150
7, Climate Change - HOT-555-0100
8, Spawn Cognoscenti (Mafia Car) - CAR-555-0142
9, Spawn Sanchez (Dirt Bike) - MBK-555-0150
10, Spawn FBI Buffalo - CAR-555-0100
11, Spawn Turismo - CAR-555-0147
12, Spawn NRG900 - MBK-555-0100
13, Spawn Comet - CAR-555-0175
14, Spawn SuperGT - CAR-555-0168
15, Spawn Police Chopper - FLY-555-0100
16, Spawn Jetmax (Boat) - WET-555-0100
there definitely has to be more. especially the character
change cheat from GTA3.