Flan's Mod is a massive mod full of customizable vehicles, guns and armor. It's the ultimate war and mechanical pack and works especially well for multiplayer servers.
It's really a mod host of sorts that contains several other mods. You can download the content packs from this link here. Content packs contain the information for the mod to read and create the containing vehicles, guns, armor and/or parts. They're like instructions for the mod to follow.
Most of the content packs will come with special crafting benches. These will contain the necessary materials to be able to put together a much larger vehicle or weapon. All you need is to have the right parts in your inventory and if the bench indicates you have the correct materials, it will allow you to craft the item. Depending on the content pack you may also be able to upgrade said vehicle or weapon with better add-ons.
You can turn your world into a first person shooter with this mod. Challenge yourself by putting the game in the hardest mode and fight off hordes of zombies. Minecraft may be notorious for not having any sort of guns but Flan's mod now gives you that option. Bring your world out of the dark ages and into modern times. Rid yourself of the need for bows and arrows and arm yourself with an automatic.
So if you want vehicles, new armor, guns or even battle mechs, Flan's mod has a seemingly endless supply. This mod will definitely take some trial and error to get used to but if you have a flair for mechanics and want to apply that to Minecraft then this Mod is worth it. This mod will take crafting to a whole new level.