PSX hacking is very easy with some powerful hacking tools. I would suggest the Xploder with the original DOS utilities (run the tool under
DOSbox on Windows XP). Alternatively you could use an emulator supported by Renegade 64, which isn't as powerful as the Xploder tools, but contains more than enough to hack most codes.
GBA is very easy to hack using Visualboy Advance.
Older Gameboy games can be hacked well enough with
BGB.
GCN hacking can be done with GCNrd + any method of running homebrew programs on GCN hardware. (PSO, or AR+SD adapter [recommended]) A broadband adapter is required.
PS2 ... I don't know. I remember seeing some sort of serial I/O hacking tool a few years ago (for PS2 EE-SIO), but I do not remember the name of it.