A game called Neverwinter Nights, however, looks to raise this bar even more. Neverwinter Nights is a Dungeons & Dragons game (literally - it's a licensed title) from the makers of the Baldur's Gate series (also a D&D game). However, whereas Baldur's Gate was a single/multiplayer game where players all played together, the multiplayer portion of Neverwinter Nights (there's going to be a similar single-player element as well) will have one character who is the "Dungeon Master" (like the pen & paper D&D game) and the other players are the characters themselves. To this end, while I believe there are no plans to allow people access to the source of the game, the amount of support for user created content is unprecedented. Bioware has been writing and publishing modeling tutorials for over a year now and just recently released the beta version of the Aurora toolset (all 241MB of it) that they have been using to develop the content of the game.
The part that makes this all interesting is that Neverwinter Nights hasn't been released yet. It hasn't even been finished yet, nor does it have an ETA for when it will be.
So I downloaded and started playing around with it and it's a very good toolkit for creating content, though they stress that there is a chance that something will change before they ship the game and that anything created with it won't work in the game, but if they don't "break" this editor before they ship the game then they stand a good chance of having user created content available as soon as the game ships. It's tricky but intuitive. One of the things that makes level editing a pain for most games and for most people is the fact that nothing is assumed - you want a level to look a certian way, you have to do everything yourself. This level editor, however, does a lot of the work for you. If you make a door, it makes another room, as well as all the changes that come along with that change. There are some who would say this "dumbs down" the process and makes it to where any idiot can make a level/adventure, this is precisely the point.
One of the ideas behind Dungeons & Dragons was that of customization - write your own adventures, etc. Your local comic book store sells graph paper for this reason. However, Bioware is trying to make this the most customizable game ever. The recent RPG Morrowind has a pretty good editing tool for it that you could make a whole new game with and this is good, but it's not what people really want. People really want to make small contained adventures - and this is what the Aurora toolkit affords them. Instead of making a game and throwing tools out there randonly or making a game that is supposed to be done and making the users finish it off, Bioware is making a product with the intention of being a platform from day one - like what id Software wound up doing with Quake 3, but more overt about it.
The bottom line for me is that if playing this game is nearly half as much fun as it is designing it, it's going to kick serious ass.