Relational Database Options that work with .Net framework aside SQL (Possibly using Linq)?

Exactly what the title says...........any thoughts on other good options for relational database implementation aside from SQL Server DB's.

It's necessary for the .Net Framework to be able to communicate with it also.

From stackoverflow

• Oracle, Postgres, MySql... the list goes on...

  .NET has native libraries for communicating with SqlServer but you can connect to any DB that has ODBC/OLEDB Support under windows.

• Depending on what you need it to do, you could use Oracle, Firebird SQL, SQLite, MySQL, BerkeleyDB or a good few others.

• Plenty of those: Firebird, PostgreSQL, SQLite, MySQL to name a few.

  Don't know about LINQ support over there, though.