Comprehensive playlist of short and witty animated videos that quickly explain the basics of D&D, including alignments, races, classes, and being a DM.
5eTools is an extensive database for both players and DMs alike. 5eTools has an excellent search function for quick reference such as item names to fill up that merchant shop you just improvised into the world, as well as comprehensive toolkits for building your character sheets from the bottom up.
World Anvil is a database much like Roll20, filled with maps, tokens, and even sheets for all of your NPC's and monsters alike. With a few more bells and whistles (such as the campaign management and writing software), World Anvil is a helpful tool for DM's looking to expand outside of Roll20 or even D&D as a whole.