Neverwinter Nights is an amazing fantasy computer game that comes as close to reproducing traditional pen-and-paper Dungeons & Dragons as is probably possible. The original game has a large single-player adventure and there are two official expansions available (Shadows of Undrentide, and Hordes of the Underdark) which add new content and options, and each also has a new adventure. For those of you who like to play RPG computer games by yourself this is pretty great.
There are several features however which make this game even better.
First, you can make your own adventures. Just like the original D&D. There are literally thousands available for download from Neverwinter Vault.
Second, it supports multiplayer. So not only can you play a cool version of say “Slave Pits of the Undercity” or “Keep on the Borderlands” but you can also get a bunch of friends together and go through these modules together. And just like Dungeons & Dragons, someone can be the Dungeon Master (DM). There’s a client program that allows you to control things behind the scenes so that you can make the adventure even better for your friends.
Finally, it is available for Windows, Macintosh and Linux. So anyone on almost any platform can play together. The only downside for Linux and Mac people is that the toolset that allows you to make your own adventures was not ported to those platforms. However there are many intrepid individuals in an organization called OpenKnights who are busy writing software to address this problem. You might check out NeverEdit.