I stumbled onto this free tool recently and thought I would add it as an addendum to my previous posting Many to Many Relationships with SharePoint Lists. The CodePlex tool doesn't simply emulate a many to many relationship like the technique I documented within my previous posting, it actually exports the data to corresponding Sql Server tables which will enforce the relationships. I haven't had a chance to actually experiment with the tool, but the literature indicates it maintains an ongoing synchronization between the originating SharePoint lists and the Sql tables. Obviously there are some potential performance implications with regard to how and when the synchronization occurs, especially when dealing with large lists/tables.