This includes very useful basemaps, showing streets, topography, cities, borders, aerial imagery, etc. that you can overlay your data on.
There are also a number of fun and artistic basemaps available, such as the heavy metal themed Spinal Map, artistic water color maps, old-timey pioneer themed maps, and others.
You can see a live version of about 20 of these basemaps at https://millermountain.com/basemap/.
Learn how to use all of these and more in your leaflet maps by watching this video.
This video is lesson #24 of approximately 65 from a course on Leaflet programming that will be available June 15, 2017. Information on that course will be available at the end of the video.