Broken Steel - Can't imagine the game without it.
Point Lookout - Works like a big added area you can visit and leave at your leisure (though it would get expensive to do it repeatedly before finishing the main quest line). But it tends to drag. I didn't care for the foggy, murky, *ugly* ambiance. There are some entertaining tasks along the way, and you get one of the best perks in the game, thanks to a bug that makes it apply to all enemies, not just one type in particular.
Operation: Anchorage - Sim within a sim. Complete linearity plus inability to pick up random loot makes it too rigid and perfunctory. High point: normal outdoor color palette! Woohoo! A welcome break from yellow and green.
The Pitt - Interesting. You get to work your way up from nothing, which I enjoyed. All you're carrying gets taken away when you get there. You get it back later on. Hunting for steel ingots is very tedious.
Mothership Zeta - I enjoyed this. Well done, with a cast of interesting characters, and as K-man said, you get some great new stuff.