GTA IV Xbox & PC File Editing Programs
Use this to open the GTA IV .rpf and .img files, it also lets you extract and replace files within the archives. This will be needed to edit or make your own custom mods.
Alternative to OpenIV and will also open up GTA IV .rpf and .img files and will let you extract / replace files.
Scripting / Coding
Use this to code GTA IV scripts, save them as .c or .cpp
ScoCompiler GUI v1.2
Use this to compile .c or .cpp to .sco
Use this to compress your .sco files
SCO Tool Box 1.2.7
Use this to view and edit .sco files
XMC 6.0 (The Best GTA 4 Mod Menu for Xbox 360/Pc)
Video (Xbox 360):