Tile5 is a HTML5 mobile-optimized mapping and tiling library, that has been built from the ground up to support multiple mapping providers.
If you are interested in mapping, I would start by having a look at the T5.Map class and if you are looking for more generic applications of the library then either at the T5.Tiler or T5.View classes.
For more information see the tile5 website: www.tile5.org