Neverwinter Nights is a game that was originally published in 2002 by Bioware. It had 2 major expansion packs as well as some smaller premium modules, three of which wound up being compiled into an expansion pack and subsequently sold as part of the Diamond Edition version of the game. The official campaign, the game as it was originally shipped, was pretty long, and not really that good. Lots of pretty generic “go explore in all four directions to complete this area” type stuff, although it definitely had some high points. The expansions were fabulous, though, Hordes of the Underdark especially.
What really made Neverwinter Nights special was none of that, though. None of it’s actual content was that important next to the massive amounts of fan-made content made possible by the release of the Aurora engine with the game, that allowed pretty much anyone to be able to make an entire game in the module creator, which could then be played through a menu option when you started Neverwinter Nights. I used to use it to make cheating mods, but a lot of other people used it to make really interesting stories and games. (I would make cool looking and super OP items, then I would create a character and give them the items. Then I would save the character before loading the character into a new game. If the game had a start that took away your items, I would edit the actual game to stick the items in.) There were hours and hours of games you could play if you had Neverwinter Nights because they were online for free download.
Until IGN took down the NWN Archive and a lot of that stuff was lost and separated.
Another thing that made the NWN player experience unique is that there were tools available that made it possible to host a long-term multiplayer world with a DM, which I know were pretty popular for a while, although I’m not sure if any are still going strong today.
Mods have become very popular in the last 10 years, but even in games with a huge modding scene, nothing has really measured up to the amount of creative content that was being produced for free by the NWN community for a while. Hopefully something similar will happen sometime soon.