TModLoader

tModLoader is a free Modding Tool that is developed by the TML team and released by them as a standalone program and can also be obtained on Steam as a DLC. It can be used to download Mods from their database, called the "Mod Browser", receive updates on Mods if there are any, or upload your own Mods to the Mod Browser. A wiki with information about creating Mods and using tModLoader can be found here.