(I'm not including the Community Expansion Pack here - I think everyone knows about that one.)
First up, there's the Castle Construction Set. We would have had real trouble making BloodSpell without the vaulted celings, towers, and fortifications enabled by this new hakpak. It's a little awkward to work with at times (the roof gets in the way in the toolset), but the visual quality's superb. Check out some of the screenshots to see what we mean. It would be even nicer with an English .Set file for naming, if anyone felt so inclined...
Second, there's NWAlleys. We end up in alleys quite a bit in BloodSpell, and there's really nothing quite right in the main game content. Fortunately, these little twisty, windy, dirty areas do the job just perfectly.
Finally, we've got the Alternate City Texture. NWN's clean and pretty city is perfect for the Church-controlled areas of BloodSpell's sets (we've got a just stunning cityscape that you only saw briefly in the trailer), but when we're getting into the slums of the Blooded, we needed something more sinister. The ACT fits the bill to a T.