So, as the title states: You can now set your fields of view to whatever you want.
Edit: Now with source and the ability to change all fields of view – first-person walking, driving, aiming, ADS, crouching and third-person.

Edit: …which can now be disabled in the .ini file.
Tested and working on the worldwide Steam and retail/Rockstar Warehouse version as of April 18, 2015 – there’s no guarantee a future patch won’t break it, but this mod will be updated for compatibility if necessary.

Be advised: We do not yet know the game’s anti-cheating measures – use at your own risk. This mod modifies game memory and as such looks like a cheat, and may even be considered one in online multiplayer as having a higher field of view can put players at a competitive advantage.
Edit: I should probably point out that I consider the risk of getting banned for using this about as high as for things like SweetFX (which “look like a cheat” in the same way), and if you do get banned, it’s most likely a mistake (though we have no idea whether Rockstar would unban you then either). I just felt the need to mention that it’s not 100% risk free.
Download and review : Field of View in First Person Mode – Gta V Mod custom FOV