http://www.fallout3nexus.com/downloads/file.php?id=3157
...this mod is just incredible.
It basically turns the game into a "World" landscape.
That way modders can create all the locations they want, and using this mod as an "interface" type of thing, they can place their location anywhere in the "globe" in the PIP Boy. Basically, it'll bring all the places that modders create, and give them the ability to literally make it one giant world where you can travel between locations like the old Fallouts.
New Pip Boy:
http://img40.imageshack.us/img40/1460/fo320090517142656.png
The Ocean:
http://img32.imageshack.us/img32/306/fo320090517142927.png
Here's another cool one. Called Existance 2.0.
http://www.fallout3nexus.com/downloads/file.php?id=5612
Another fantastic mod, adds a robot dj radio station full of ambient music with a quest only doable by listening to the robot's lucid dreams.
Fits perfectly and immersively in the Fallout world.