The Torchlight editor. This allows you to create custom content for the game, such as maps.
We are reminding you that you'll need .Net installed if you want to use the editor. The first time you run the editor it will take longer to start up than it should on subsequent launches. We're shipping the raw assets, so the editor has to go through and compress all of them the first time it runs. There are also some C# assemblies that have to be CodeDOM compiled the first time which takes a little bit.