I am current playing the first Diablo. Except I'm playing a remake of it that was made using the Neverwinter Nights toolset. It's pretty awesome and a huge amount of attention to detail was put into this. They were even able to include all the music and voice files from the original game. From that familiar music that plays in the town to the stuff that plays in dungeons. It gives it a lot better atmosphere when the music and the NPC's sound just like they did in the original.
The mod is well over 30 hours long and it took 2 years to complete so the people who made it put a ton of work into it and it's not like they were getting paid for it. My character is a sorcerer and I just barely was able to defeat "The Butcher" earlier today.
Mods like this are one of the reason the NWN community is still around today even though the game came out in 2002. Yet you have people still making new mods for this game to this day, some of them staggering in scope. It essentially gives the game almost endless replay value. Not only do they have custom modules, but there are also mods made for things that already exist. I found a mod for the tv show "Firefly" the other day.
There are some pretty cool mods that take place in the Lord of the Rings universe and also the Dragonlance universe. People have also turned several tabletop adventures into mods. There is a "Tomb of Horrors" module. There are mods based on the "Eye of the Beholder" series. Some of the people that worked for the company that made NWN also worked on some of the fanmade mods, though unofficially and for free. There are also mods called "Persistent Worlds" that are basically multiplayer mods that can only be played online. Some of them still have a lot of people playing, but I never play them because I don't like playing games online with people so I stick to single player mods.
If anyone is interested you can get mods from here, it also has mods for NWN 2, but I don't play the second one was much as I play the first one:
http://neverwintervault.org/
I just recently completed this mod, which in it you play as Khelben "Blackstaff" Arunsun, which for those who don't know is a powerful wizard from the Forgotten Realms universe and a peer of Elminster.
http://neverwintervault.org/project/nwn1/module/perchance-dream