The list of supported formats can be split up in 2 categories: vector data and raster image data. All are accessed through so-called LayerModels, which are abstractions capable of accessing of the data underneath.
Vector based layer models:
Geotools : support for shape file, WFS (Web Feature Service), PostGIS, DB2, JDBC, ArcSDE, ...
Hibernate Spatial : support for PostGIS, Oracle, MySQL
Raster images models:
WMS : Support for Web Map Service.
Google : Supports the Google Maps (default, satellite and hybrid).