Note: Neverwinter went gold on June 11th, 2002. It's somewhat different from what was expected, at the time I originally made this writeup. A disclaimer; I'm one of the Neverwinter Community Official Moderators (BioMods).
Neverwinter Nights is a Dungeons and Dragons 3rd Edition RPG from Bioware Corporation. It is a pseudo-MMORPG, allowing play in Single Player, small-group multi and full-scale MUD-like worlds.
NWN's features include:
Post-release, I can say that NwN is not the holy grail that I once held it to be. However, it is still a very good game. Perhaps the largest complaint is that the game is really designed for one character, as opposed to Baldur's Gate's six. A henchman and a summonling can be added. Another complaint is the single player campaign that ships with the game-- it is widely considered to be subpar, compared to BioWare's other offerings. However, there are currently nearly 2000 user-made modules, and BioWare has started a free campaign, called "Witch's Wake" that has received much praise.
In addition to the "Neverwinter Aurora Toolset", BioWare has released a "plot wizard", to further simplify the adventure creation process.
More info can be found at the Neverwinter Community 'Site (nwn.bioware.com), Neverwinter Vault (nwvault.com) is useful as well.