OpenStreetMap (OSM) data is free but its own servers have limited capacity.
There are many other organisations that provide map tiles made from OSM data. In this demo you can switch among different tile servers.
Available tile servers in this demo:
- standard
- Wikimedia Maps
- Open MapQuest
- OpenCycleMap (from Thunderforest)
- transport map (from Thunderforest)
- Stamen Toner
- Stamen Watercolor
- CartoDB
Full list of tile servers is available at: http://wiki.openstreetmap.org/wiki/Tile_servers
Disclaimer: I hope I got all the attributions right.