For all you passionate GTA V researchers out there, a few tools for your batman belt
Source File: data/lang/american.rpf
Description: English language strings, subtitles, etc.
Note: Every textual string shown in the game should be here
Source File: common.zip
Description: Internal files listing common things like all vehicles, all missions, etc
Note: Contains a ton of internal game data, great for finding secrets
Source File: levels.zip (part 1)
Description: Content of the levels themselves (what's on the map)
Note: Contained binary files like textures, since we can't view them all, filenames and strings
Source File: levels.zip (part 2)
Description: Detailed game objects like vehicles, scenes, props (what's on the map)
Note: Also contained binary files like textures, kept the filenames and strings only (for searching inside)
Disclaimer: These files are intended for gunters. They contain tons of spoilers. If you're not obsessed with murals and in-game conspiracies, you should probably skip this one.