You can use Eclipse with the [Eclipse Data Tools Platform](http://www.eclipse.org/datatools/downloads.php):

- SQL querying (with a GUI to create create queries visually)
- Can be connected to SQL Server, Oracle and MySQL.
- Can run on Windows and Linux distros
- DDL Manipulation 
- Create/edit stored procedures, triggers.
- support for JDBC connections

![enter image description here][1]

![enter image description here][2]

PS: [Toad](http://www.quest.com/toad/) is Windows only but otherwise it is a great tool and has all the features you asked for (except OLEDB / ADO.NET for which I am unsure).

  [1]: https://i.sstatic.net/gJe71.png
  [2]: https://i.sstatic.net/6Cogv.png