Somone asked about world-building resources on a forum and it brought back some pleasant memories. I started building worlds as a dungeon master in the Advanced Dungeons & Dragons role-playing game system many years ago. Their book, World Builder's Guidebook, is still one of my favorite resources for creating worlds, especially in a medieval fantasy setting.
I ran a game for almost ten years with the same players. Their characters and mine lived out fantasy lives in a world full of magic and adventure. Many people have a poor opinion of Dungeons & Dragons, but what they never seemed to understand is that the dungeon master is in control of how the game plays out because he/she creates the world. You can have a game filled with evil, death and greed, or you can have one based on honor, love and personal growth. It's all in the mind of the DM and players.
It was a lot like writing with several collaborators. I'd create the setting and overall plot and the players would interact and change the direction of the action based on their actions in the game.
I think that's why I'm writing now. I missed the thrill of creation.