mirroring databases

Has anyone done, or know of utilities for, remote location "near real time
mirroring" of postgres databases?

An update every 15 minutes or so would work for this purpose. However, the
total number of records is in the millions, and the number of changed or
new records per day could be thousands.

To avoid transferring all records, I'm imagining a flag column that
indicates if a new or changed record has been transferred to the mirror yet.

Any suggestions...? 

- April