I was wondering if anyone had a definitive list for add-ons, especially something like an "unofficial patch". I have made only one add-on myself, to make all the traveling merchants immortal. They had a tendency to get wiped out by albino rad scorpions. But it's been a while since I have played, and I'm thinking of starting a new game.

That's a mod (modification), not an add-on (DLC).

As for official DLC, anything but Mothership Zeta
As for mods, well, the list is too big. But I personally like my own mods best, obviously, though their released versions are far behind from what I have personally. I've grown quite fond of my Enclave mods.

the pitt and point lookout

Check out The nexus http://www.fallout3nexus.com/downloads/top/alltime.php