.db - Database files
Safari's cache databse is in sqlite3 format on Mac OS 10.5 and the .db extension is getting more common.
Before Mac OS 10.5 db was mostly used for Berkeley DB files.
See the general File Formats page.
Links to Berkeley DB softwareSleepy Cat - the home for Berkeley DB
There is open source software available to dump the contents of Berkeley DB files: Python Bindings for BerkeleyDB. I have not tried to build this for Mac OS X.