Dungeon Masters Guild (DMsGuild) is an e-commerce website that sells D&D modules and supplements. They have a large library to choose from with some content provided for free. Their network of websites was founded in 2001 by James Mathe and include DriveThruRPG, Dungeon Masters Guild, DriveThruCards, DriveThruComics, DriveThruFiction, and Wargame Vault. You can read about their full history here.