Torchlight 2. Dubbed "GUTS," the suite of tools allows the user to change everything "from item and skill balance, to level layouts, animations, quests, and much more," according to the game's official blog.
Mods created with GUTS can be shared online and also support multiplayer, sessions for which can be found in a separate "modded games lobby." The tool also includes a utility to help convert pre-existing mods into the new GUTS format.
As part of the GUTS addition, Torchlight 2 now features full Steam Workshop integration, allowing users to subscribe to (and subsequently receive automatic updates for) the mods they're particularly fond of. The full GUTS wiki can be found here, along with instructions for its installation.