What database are you using?
With MySQL, I've always used the database itself to do any replication/mirroring. You could do something as simple as dumping the information you want from DB A with
mysqldump and then loading it into DB B.
Update: Just noticed your P.S. :)
You could also setup up MySQL Replication
If you're not using MySQL, take a peak at the docs for whatever DB you are using.