OpenAtlas

Evaluate WMS or similar map providers without restrictions of requests

from leaflet providers plugin:
https://github.com/leaflet-extras/leaflet-providers

Some examples without the need of an API key (However, we should check if they track the requests due to legal issues, cookies etc.):

var OpenStreetMap_Mapnik = L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    maxZoom: 19,
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
});

var OpenStreetMap_HOT = L.tileLayer('https://{s}.tile.openstreetmap.fr/hot/{z}/{x}/{y}.png', {
    maxZoom: 19,
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors, Tiles style by <a href="https://www.hotosm.org/" target="_blank">Humanitarian OpenStreetMap Team</a> hosted by <a href="https://openstreetmap.fr/" target="_blank">OpenStreetMap France</a>'
});

var OpenTopoMap = L.tileLayer('https://{s}.tile.opentopomap.org/{z}/{x}/{y}.png', {
    maxZoom: 17,
    attribution: 'Map data: &copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors, <a href="http://viewfinderpanoramas.org">SRTM</a> | Map style: &copy; <a href="https://opentopomap.org">OpenTopoMap</a> (<a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA</a>)'
});

var Stadia_OSMBright = L.tileLayer('https://tiles.stadiamaps.com/tiles/osm_bright/{z}/{x}/{y}{r}.png', {
    maxZoom: 20,
    attribution: '&copy; <a href="https://stadiamaps.com/">Stadia Maps</a>, &copy; <a href="https://openmaptiles.org/">OpenMapTiles</a> &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors'
});

var Stadia_Outdoors = L.tileLayer('https://tiles.stadiamaps.com/tiles/outdoors/{z}/{x}/{y}{r}.png', {
    maxZoom: 20,
    attribution: '&copy; <a href="https://stadiamaps.com/">Stadia Maps</a>, &copy; <a href="https://openmaptiles.org/">OpenMapTiles</a> &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors'
});

var Stamen_Terrain = L.tileLayer('https://stamen-tiles-{s}.a.ssl.fastly.net/terrain/{z}/{x}/{y}{r}.{ext}', {
    attribution: 'Map tiles by <a href="http://stamen.com">Stamen Design</a>, <a href="http://creativecommons.org/licenses/by/3.0">CC BY 3.0</a> &mdash; Map data &copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors',
    subdomains: 'abcd',
    minZoom: 0,
    maxZoom: 18,
    ext: 'png'
});

var Stamen_TerrainBackground = L.tileLayer('https://stamen-tiles-{s}.a.ssl.fastly.net/terrain-background/{z}/{x}/{y}{r}.{ext}', {
    attribution: 'Map tiles by <a href="http://stamen.com">Stamen Design</a>, <a href="http://creativecommons.org/licenses/by/3.0">CC BY 3.0</a> &mdash; Map data &copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors',
    subdomains: 'abcd',
    minZoom: 0,
    maxZoom: 18,
    ext: 'png'
});

var Esri_WorldImagery = L.tileLayer('https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}', {
    attribution: 'Tiles © Esri — Source: Esri, i-cubed, USDA, USGS, AEX, GeoEye, Getmapping, Aerogrid, IGN, IGP, UPR-EGP, and the GIS User Community'
});