Neverwinter Nights is an old flame of mine. In actuality, I played the game for over 2 years. While never touching the single-player campaign (the game wasn't even made with a single-player RPG in mind, so Bioware ended up threwing in a half-assed rpg), I was fascinated with online play, Dungeon Master modes, and NWScript.
Online play in general on NWN is so diverse it can please everyone but the hardest of the hardcore Pen and Paper players. You could find the ultimate hack and slash campaign of your dreams that can take you from level one to level sixty, or you could play a true-to-the-game Dungeon Mastered game with absolute realistic human engineering.
The tools used to create such fantastic stories themselves are, in-fact, appliable to the DM client. Originally, Neverwinter Nights came with a very crappy Dungeon Master system in-hand. You merely had the ability to create/kill/invulnerabize/mortalize/possess/unposess/send away monsters, create items, and if you were skilled you could find a way to change base stats (Experience, STR, DEX, CON, ect). This was very frustrating to the community, so they eventually engineered a more powerful dungeon master system, the one I worked under for most of my gameplay experience. It allowed me to change the music, play sounds, puppeteer NPCs, change weather, instantly buff a monster, and many more useful additions were included. The DMFI tools are still available for any DM to use, and yes, players appreciate their use.
Most of my time was spent programming NWScript in the toolset. NWN's toolset, Aurora, included a lot with the original game and its expansionss, but with the community expansion pack that amount was exponentially increased. Just before it came out I was working on a massive conversion of all the D&D creatures I could find into NWN while keeping them as closely to the context in the books as possible. This included a lot of interesting programming, and some of my finest achievements were pronounced with my first major project, the Chaos Beast. In itself, the Chaos Beast is as similar to "The Thing" as it gets. Every 6 seconds after spawning (used spawn script, not heartbeat), the creature would change shape into many other creature types and would emit a contageous aura. This aura would infect both humans and NPCs, and effectively players and NPCs alike would eventually go insane and become Chaos Beasts.
I could go into a lot of the things I've done... but that's just one of my favorite things. I also worked greatly on converting BGII to NWN (elementally, not cloning), such as creating random weapon and clothing spawns on certain creatures along with loot dropped in bags a little way from an actual corpse that would eventually rot. I also made it so that anything dead could effectively be toyed with, so to speak. You can burn bodies, skin them (animals especially), eat, sacrifice, pray for, bury, and many other scripting proceedures...
... Sorry about the rant.
To LadyM
If you got the entire platinum edition, play Shadows of Undrentide and move into Hordes of the Underdark. Those two are actually connected games, but the original NWN is not connected to anything; not to mention it's just a terrible game to begin with. =)
However, I enjoyed SoU and HotU was just great.
BGII is THE fantasy rpg -- but it's getting older every day. If you have any spare change, it's worth every penny.