Already played around with some great fan mods.

One allowing you to call in a Vertibird for air support or to drop off a squad of Enclave soldiers for you to command.

Of course, the nude mods, heh. These are always the first to appear. Fans never cease to amuse me.

Many companion mods adding new NPCs to aid you, or just giving existing NPCs support to join you.

TONS of new weapons and armor.

Graphical updates like larger textures with more detail. These tend to bring even the beefiest of systems to a halt (I have a P i7 w/ 3gig DDR3 at home and it came to screeching halt on many of these).

Gameplay tweaks for weapons, creatures, NPCs, UI.... many of which alter balancing for easier or more difficult gameplay.

Many, many more. I have fun on weekends just sitting down and downloading a few to check them out. Some great stuff out there.