I'd disagree with Tibby's statement anyway. Before I even played the game the first time I installed darnified UI, that's a must have. It doesn't change the game technically, it just makes the menu's displayed prettier and more efficient.
http://www.bethsoft.com/bgsforums/index.php?showtopic=984736
Apart from that, theres heaps at fallout nexus as LividDK says. Theres a lot of good ones.
I highly recommend FOOK 2, it gives you (and the enemies) a whole bunch of new equipment, and it randomizes their loot so it's rare to see two guys wearing the exact same thing'
Also highly recommended is Mart's Mutagen Mod, it randomizes the enemies stats a bit and changes their ai. So if you found 2 wild dogs or whatever, then one might be a bit bigger, braver, and dark brown, while the other is more likely to run, smaller and grey.
A personal fave of mine is the sprint mod, it just gives you a sprint key that uses VATS. I also use a slowmo mod.... come to think of it, I never use VATS for VATS anymore.
Theres heaps more, but that's an excellent start =]
Oh, I also recommend looking up how to tweak your game (done via editting the ini) so you can get the best bang for your buck.