db4o is an object database for Java and all .NET environments. Its features include ACID transactions, automatic class schema recognition, query-by-example, a S.O.D.A. object querying API, native queries, LINQ support on .NET, callback methods, memory files, cascade-on-delete, multi-threaded access, and Java Webstart and servlet support.
dbtcp is a package of libraries which provides applications on Linux access to ODBC datasources on remote machines running Windows. It allows a Linux application to access data held on a Windows machine (such as MS SQL Server on NT), and includes a command line client, a DBD, and a PHP driver.