Schema migration tools?

Greetings,

We have the traditional three servers:

    dev --> staging --> production

each with a PostgreSQL instance and the same schema, at least over
time.  Right now, we handle schema migration (updating the schema for
new releases) by manually-created scripts that apply the changes to
staging and production.

I'd like a tool that would automatically create these scripts, and I
wondered if anything like this existed.  The theory would be that it
would consider two databases a and b, and produce the appropriate
script to change b's schema to match a.

Does anything like this exist?  If not, I might have a new project...

Thanks!
-- Christophe