r/battlemaps – Find and share battlemaps.
Matt’s Massive Map Menagerie – Google Drive dump of battlemaps and different mapping assets.
Imgur Map Album – An album of maps!
Dungeon Channel Map Collection – Some free maps and other assets.
Forgotten Adventures – u/Stryxin‘s Patreon page. Makes high quality maps available for free with additional content available to his supporters. Here is a free archive of his older maps, and his imgur where he puts all his free content available on Patreon.
Syrkres – Patreon page of great maps.
Blue Sword Games – Patreon page of high quality maps and assets available to anyone. Additional content available to supporters.
MapHammer – Patreon page, most content locked for non-supporters. Extremely high quality work!
Elven Tower – Patreon that creates full adventures and battlemaps to go with them. Most content is locked for non-supporters.