Long term schema management

[Dave Dyer]

We have MYSQL based product which has many databases on many sites, all sharing a common
(but evolving) schema. It's becoming increasingly troublesome for the support staff to be
sure that each database's schema is up to date and paired with an appropriate version of
the software that manipulates it.

Up to now, we've worked by maintaining an official schema prototype, and periodically distributing
MYSQL command files (effectively database patches) which update existing schema to the new
schema. It's clearly past time to do something a little more formal.

My question; what tools already exist which could keep track of the schema revisions
and selectively apply them to a gaggle of databases.