Depends on what evel you are talking about.


1). Object level - Should be version control of DDL in DB, CVS, Source Safe etc.
2). Schema Level - I think TOAD and OEM does a schema comparison (also for object level also). A bit of a bitch though when your database contains 150 schemas.
3). Database level - init.ora paramters etc, select statements through unions and db links is one way.
4). OS level - YOu'd have to write scripts to determine set up of boxes. You'd rarely find two boxes identical.