Neumont ORM Architect (NORMA) for Visual Studio is a free, open source ORM tool now released as a "community technology preview". This release has a diagram editor that supports the new ORM2 notation to include mapping of the ORM schema to 4 RDBMSs (MS SQL Server 2005, DB2, Oracle, and PostgresSQL). Read the rest of the post for what's new with this release.

Note: NORMA is an add-on to Microsoft Visual Studio 2005, so you need to have that installed before you can use it. You may also need to first install PliX (http://sourceforge.net/projects/plix) and the Visual Studio 2005 SDK version 3.0 (http://www.microsoft.com/downloads/details.aspx?FamilyId=7E0FDD66-698A-4E6A-B373-BD0642847AB7&displaylang=en) to get the current version of the domain specific language tools.

I'll update this post with any release notes once they are posted on Sourceforge.