I only traffic in Game Shark codes that affect wild pokémon, which you can then capture and train. Also, be aware that the two versions require completely different sets of codes; these are from Ruby. If these don't suit your needs, check GameFAQs.
Wild pokémon have Max IV: D95E6845 60A3B6CF
These codes set the natures of pokémon you encounter in the wild. Note that they have no effect on bred pokémon or pokémon recieved as an event.
NUETRAL NATURES
Bashful : 38F8B9C9 607FE8FA
Docile : DDD50730 6ED65341
Hardy : 85C533B4 E777280C
Quirky : C1167B1B 2998D2D4
Serious : DE9BD9AA B875822B
NATURES IMPROVING ATTACK (and favoring red Cool Pokéblocks)
Lonely : 0CAF6A9B EA27BA84 (low Defense)
Brave : A7087215 F874A980 (low Speed)
Adamant : A42CC56D 3DF4DF4C (low Spc.Atk)
Naughty : 89224E28 B0D0928C (low Spc.Def)
NATURES IMPROVING DEFENSE (and favoring yellow Tough Pokéblocks)
Bold : 9BA2EA7F BF343E77 (low Attack)
Relaxed : 5BA86306 364B284B (low Speed)
Impish : 77B82909 4D321507 (low Spc.Atk)
Lax : B96C8F98 0529BB0A (low Spc.Def)
NATURES IMPROVING SPEED (and favoring pink Cute Pokéblocks)
Timid : 6B72D649 A850346C (low Attack)
Hasty : A52EFEF5 5506A2C4 (low Defense)
Jolly : EDA3BC0C E65B1542 (low Spc.Atk)
Naive : 09F6BE91 6A2662C2 (low Spc.Def)
NATURES IMPROVING SPC.ATK (and favoring blue Beauty Pokéblocks)
Modest : F28DD676 EECDC5A1 (low Attack)
Mild : 64639370 D15C4427 (low Defense)
Quiet : EB935861 595B8DB6 (low Speed)
Rash : A7A5DD65 37566B3B (low Spc.Def)
NATURES IMPROVING SPC.DEF (and favoring green Smart Pokéblocks)
Calm : 049C1F3F 2F043BFB (low Attack)
Gentle : F3D8252C 9916EF61 (low Defense)
Sassy : F2C2B934 D5B6C8AE (low Speed)
Careful : 531ADA3E 040B6679 (low Spc.Atk)