SQLite is also an excellent option for a datastore on OSX.Its not nearly as full featured as postgres but no application is required and as you have a OS independent file per db which is extremely portable.SQLite Professional is a relatively decent free gui you can use also.
SQLite has limitations on the data types it supports. Most of this can be worked around by application code, but it can be a pain when you have data that needs to be accessable by more than one application.
