Schemata is a gem for corralling schema definitions across projects. As we work on new projects a lot of common field requirements come up ( addressess, contact forms, medical forms ) but its rare that the field names are ever implemented the same. Its a minor issue, but a major pet peeve of mine is when 'email_address' is defined as 'email' or 'phone_number' defined as 'phone' (Don't get me started on street_address_1 and all of its relatives ).
So with schemata you define the fields once and then they are available to use in your migration files.
gem 'schemata'
This is going to be very interesting to watch from a Search perspective. Google will have to come up with a similar rel= con type of system...i think.