help with finding a data dictionary...

Can anyone recommend a 3rd party data dictionary. We want to create
databases using SQL and have them as portable as possible. Ideally this
3rd party app could have the abiloity to add a database from one that
already exists and be able to auto convert databases if not in sync. I
would appreciate advise form anyone who has experience in this area or
anyone who can point me in the right direction

Thanks.

Tom.