I remember this game.

Didn't the plot kind of... stink, a little?

Is it better with the expansions?

Yes, the OC was pretty bad. The real value is the the ability to play community-made mods. The expansions are fine, they're just shorter than the OC.

Shadows of Undrentide had a pretty good/exciting plot,
and Hordes of the Underdark had a really good and plot/scripting. Some of the battles and puzzles were beastly, though.

As for the premium modules:
Kingmaker - I've heard was good.
Witch's Wake - not so good. But, from a Pen&Paper perspective, it was noteworthy.
Darkness over Daggerford (not quite a premium mod) I've heard was quite good, with cut scenes and everything, although I've not played it.

I can recommend
adamandjamie.com/mod/nwn_downloads.aspx (Adam & Jamie's mods) specifically Demon (very twisted), Dreamcatcher, and Shadowlords.

Just head over to nwvault.ign.com

Someone mentioned something about there being a free version of Endless Dungeons.

Hope this list is useful.

Yep, just go straight for the user-made modules. The top-rated can be found here: http://nwvault.ign.com/fms/TopRated.php?content=modules&lang=en&sort=&dir=ASC&w=&p=1

There's something for pretty much any RPG players taste there (class-specific modules for example), and plenty of the games aren't too long (like Almraiven, a wizard/sorcerer module, which is 6+ hours), so you have plenty of time to enjoy more than one. Though as good as the custom content is, I do enjoy Shadows of Undrentide enough currently that I plan on also completing both that and HotU.