As quoted from the website, SubSonic is a toolset that helps a website build itself. Basically, it generates the DAL (Data Access Layer) on the .NET platform for you.
Currently SubSonic supports MS SQL Server, MySQL and Oracle.
I particularly like the design concept using ActiveRecord. SubSonic uses the ActiveRecord pattern for the data-aware base class for each table object. This class implements Generics, which allow for a great reduction in code and a lot of flexibility.