Some tile providers allow the usage of the tiles they render directly. But these providers, usually recommend limitations on your usage in order for avoiding excessive bandwidth and resources.
MapTiles takes advantage of their tiles favouring their usage recommendations.
Maptiles is a free tool made for myself mainly.
What tilesets are available?
The layers shown here are some of the layers mentioned in OpenStreetMap’s Tiles wiki article.
OpenStreetMap Mapnik Style
This is the default style shown on http://openstreetmap.org. These tiles are cached directly from the OpenStreetMap Foundation’s .tile.osm.org servers.
OpenStreetMap Mapnik Grayscale Style
This style is a grayscale version of OpenStreetMap’s Mapnik style. These tiles are cached from the Wikimedia Foundation’s tiles.wmflabs.org servers.