Torchlight 2 editor, GUTS, released with Steam Workshop integration
Using the tool, you'll be able to modify almost anything in the game, "from item and skill balance, to level layouts, animations, quests, and much more." Furthermore, your GUTS mods can be shared with your friends. Multiplayer games using mods can be found in the modded games lobby. There's also a special tool included that lets you convert pre-GUTS modes into the official mod format.
Looking to simplify the mod sharing process, Torchlight 2 now fully supports Steam Workshop. This allows for automatic mod subscription and synchronization for any mods hosted on the Workshop. For more information about modding with GUTS check out the wiki.