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Essential for many large mods
Lets you manage mods for F:NV much more easily.
Alternative to FOMM, allows you to download mods directly from the manager's website.
200+ New Weapons, including Salvaged AKs, Railguns, Laser Katanas and more! All are added to the loot/enemy tables so you'll find them on enemies and lootpiles.
Makes the lighting in New Vegas much more realistic, with brighter, less orange days and darker nights.
Adds over 100 new, difficult areas to explore and fight your way through. Expect pain.
Revamps the particle effects, textures and models for blood, gibs, energy weapons and more.
Allows you to merge Fallout 3 and New Vegas if you own both games. Prone to bugs, but compatible with most Fallout: New Vegas mods.
Dozens of new weapons, overhauls of the loot and hunger system, and a complete rework of gameplay mechanics.
If you want more, check here. It's the largest resource of Fallout: New Vegas mods.