The Witcher 2: Assassin's of Kings developer CD Projekt RED has released its REDKit suite of mod tools in open beta as a free download -- you'll just need to own the game to partake. This has been a long time coming and should result in some quality user-made stories.
I always tell myself I'll one day attempt to get into modding, but that desire never pans out in a meaningful way. It does seem that of all the games one could create mods for, The Witcher 2 would be a good choice given its toolset. Any experience tinkering with REDKit?