Ok, that makes it a little easier
There are two main Enclave quest mods: http://fallout3.nexusmods.com/downloads/file.php?id=11351 can be played with a regular game but http://fallout3.nexusmods.com/downloads/file.php?id=11587 replaces the Main Quest of FO3. I've played them both and would recommend them to anyone.
Quest mods:
http://fallout3.nexusmods.com/downloads/file.php?id=13308 expands on Trouble on the Homefront, introducing new quests and allowing you to live in the vault again if you want.
http://fallout3.nexusmods.com/modules/members/index.php?id=137055 has several excellent quest mods. In the Shadow of the Swamp is my personal favorite.
http://fallout3.nexusmods.com/downloads/file.php?id=8747- Even if you don't like Zeta, this is a fantastic mod. You'll become the head of a new faction, have beautiful accomodations aboard Mothership Zeta and your people will be deployed to various spots across the wasteland.
http://fallout3.nexusmods.com/downloads/file.php?id=10779 - introduces new lands to explore and a couple new followers
More stuff:
http://fallout3.nexusmods.com/downloads/file.php?id=5573 - Ever wondered what's inside all those boarded up buildings out there? Now you can find out.
http://fallout3.nexusmods.com/downloads/file.php?id=2761 - Hugely popular overhaul that makes the game a bit harsher and more of a struggle for survival.
Love robots? You need http://fallout3.nexusmods.com/downloads/file.php?id=712.
Want to do missions for the Regulators? Grab http://fallout3.nexusmods.com/downloads/file.php?id=15645.