Open Alternatives to Google Maps

de Slashgeo.org

Lately there was a not much surprising news about Google products and services. Among other things Google has changed the Google Maps API use policy and will charge to those users that exceed some download limits.
It is well known that Google Maps is one of the most (or the most) famous mapping service used around the net and it starts the web GIS revolution some years ago but hopefully it is not the only API we can use. Bing and the discontinued Yahoo Maps, are great competitors but there are great and open alternatives to use.
I wrote this post to summarize the most important JavaScript API alternatives to Google Maps.
[Editor's note: this anonymous submission mainly discusses OpenLayers, Polymaps and Leaflet]