Hi there

As people have already said, the Nexus is a great place to get mods from and since someone has already provided a link to the main site, I am going to provide you with the links to some of my favourite mods!
http://www.newvegasnexus.com/downloads/file.php?id=35335 - like it says, provides realistic weapons damage. Headshots are now an instant kill (although the mod I originally downloaded for this role has been taken off the Nexus, I still use this one).
http://www.newvegasnexus.com/downloads/file.php?id=34888 - the MUST HAVE mod! It recolours the ENTIRE game. I can't describe it, just take a look.
http://www.newvegasnexus.com/downloads/file.php?id=35485 - this mods the Brotherhoods Safehouse to have every Vanilla weapon and Armour/Clothing item in the game, along with respawning ammo containers!
http://www.newvegasnexus.com/downloads/file.php?id=37199 - this upgrades the Lucky 38 Suite! At first you won't notice, but wander through the rooms and you'll see a couple of new rooms added!
http://www.newvegasnexus.com/downloads/file.php?id=36922 - this lets you set up your own settlement! A popular mod in FO3, but is still in Beta for NV, but with a dedicated Modding team and an active forums on an external site, this is constantly being improved.