LINQ is an extremely useful new language feature for the .NET Framework 3.5. It allows querying all sorts of data structures via the actual programming language (thus the Language INtegrated in LINQ). In addition, the specific variant, LINQ to SQL, includes a full O/RM (Object/Relational Mapper). Since many developers spend a ton of time mapping their relational data to objects, this included tool is a phenomenal time-saver. Of course, this is not a new concept, nor is it the first of its kind for .NET. It’s just free and included!
LINQ to SQL currently only works with SQL Server 2005. It’s not likely that Microsoft will bother with any other providers with the Entity Framework and LINQ to Entities on the horizon. There is a way to get standard LINQ to SQL to work with the SQL Server Compact Edition (SSCE) however. You have to use the SqlMetal command-line tool to generate the appropriate file(s). Once you generate the .dbml file, you can even open it in the Visual Studio 2008 designer.