OpenIV
OpenIV is used as the primary RPF explorer for GTA V. You'll need it for exploring, obtaining and editing files within GTA V.
CodeWalker
CodeWalker is an World Editor for GTA V, you'll need to use it for placing your MLO's, entities and other.
Ytyp Generator
Ytyp Generator is an open source tool for generating Ytyp files and saving them to your clipboard, saving you plenty of time.