Lots of mapping (side) projects are using the free raster tile layers generously provided by Stamen Design: Watercolor, Toner (light and dark), and Terrain. These layers are popular because you can use them without creating accounts or obtaining API keys.
Unfortunately, Stamen can no longer afford to provide these for free and have partnered with Stadia Maps to continue providing them with a free tier. Stamen has also converted the Terrain and Toner layers to vector tiles.
Announcement: Here comes the future of Stamen Maps | Stamen