Metriod Prime
Cheats

Hard Mode
Beat Metroid Prime to unlock Hard Mode. In this mode, enemies attacks are more powerful than normal.
  Contributed By: Dark Jedi Knight, Source: Brady Games Official Guide
Multiple Endings and bonuses
Beat the game at 74% or less to get ending 1
Beat the game at 75-99% to get ending 2
Beat the game at 100% to get ending 3
  Contributed By: DMXFreshBlood, Source: Brady Games Official Guide
Unlock Art Gallery 1
Scan 50% of the logs in the game to unlock the first Art Gallery.
  Contributed By: LegacyX, Source: Self
Unlock Art Gallery 2
Scan 100% of the logs to unlock the second Art Gallery.
 
  Contributed By: dynamyke, Source: self
Unlock Art Gallery 3
Beat the game on hard mode
  Contributed By: Xeromagi, Source: Self
Unlock Art Gallery 4
Collect all the items in the game.
  Contributed By: Xeromagi, Source: Self
Unlock Original NES Metroid/Metroid Fusion Suit
To unlock the original NES Metroid, simply beat Metroid Fusion on GBA, then link the GBA (with Fusion in it) to the GCN (with prime in it) then at the main menu of prime goto the bottom where it says GBA link bonuses or something like that. Make sure you have beaten Metroid Fusion though.

To unlock the Metroid Fusion Suit, do the exact same thing only dont beat fusion, but beat prime.
  Contributed By: rocktiltuesday, Source: Unknown