Vehicle Controller script
You drive cars in GTA 5, but now you can truly operate them with the Vehicle Controller script. Pop the hood and trunk, signal turns, turn on the hazard lights and even the interior light. I know most of this sounds cosmetic, but there are some real in-game perks, such as the ability to leave the engine running and the driver's side door open, useful for quick getaways. You can also toggle cruse control so you don't have to lean on the gas pedal constantly during long drives. Here's a video.
Gravity Gun script
Put a little Half-Life 2 in your GTA 5 with the Gravity Gun script. Equip the stun gun, aim it at something, and hold the E key, and you'll be able to lift and hurl your target really, really, really far. Works on cars, hapless NPCs, and other objects. You can watch it in action here.
Gta 5 shipped with a fairly constrained field of view, but modder DrDaxxy got to work and produced an FOV mod. Now you can set your own custom FOV. Installation instructions are in that Reddit thread. Note: the FOV mod doesn't appear to have been updated since the last Rockstar patch, and may be out of date at the moment.
Nice Fly script
You won't look like Superman—not yet anyway—but you can fly like him. The Nice Fly script lets you sail through the air—in directions other than down—without having to steal a jet first. The mod requires you to be wearing a parachute, ironically, though you won't need to pull the cord. Because you can fly. Here's a video.
Car Cannon script
You've shot cars in GTA 5, but have you ever shot cars? Like, out of your gun? The Car Cannon script replaces your bullets with automobiles, letting you fire a stream of cars and trucks of all kinds, because why the hell not? It's Los Santos. Place is straight crazy.
North Yankton Loader
GTA 5 begins in the frigid climes of North Yankton, after which you're whisked to Los Santos for most of the rest of the game. If you'd like to revisit it on your own schedule, this North Yankton Loader is a modified version of the Native Trainer that comes with a submenu to let you race around the snowy streets whenever you like.
Ragdoll on Demand Mod
You've punished random pedestrians in all sorts of horrible, often lethal ways, but sometimes it's fun to just freak 'em out by flopping all over them with the Ragdoll on Demand mod. This mod lets you turn your limbs to jelly whenever you want (even in cutscenes). It's a good way to amuse yourself, too, especially during lengthy exposition scenes.
Mobile Radio script
GTA games always have great radio stations, but it's sort of disappointing that you can only listen to them while in a vehicle or one of your houses. The Mobile Radio script lets you listen to your favorite tunes on foot as well. Just imagine your phone came with earbuds.
Customize Plate script
You've bought (and stolen) some cool cars, but it's time to let the citizens of Los Santos know they're yours. The Customize Plate script lets you open a text pane, type in your plate, and see it instantly appear on your car. Here's a video.
Proceed to the next page for installation instructions. And warnings!