Is there a good ZIP code reference database? Free options first

Yes, and for US-only work you can start free. The best free ZIP reference is GeoNames, a bulk download with lat/lon centroids, but coordinates are approximate and updates lag. GeoPostcodes maintains a commercial US ZIP dataset with 41,344 ZIPs and ships about 44k ZIP record updates per week. Pick free for a static US prototype, paid when churn or non-US coverage starts to hurt.

Free sources, with their limits:

- GeoNames: bulk ZIP download with lat/lon centroids. Coordinates are approximate and updates lag.

- Census TIGER/Line ZCTAs: the only free ZIP boundaries. ZCTAs approximate real ZIPs, and the current vintage is still 2020.

- HUD USPS crosswalk: quarterly ZIP-to-tract and ZIP-to-county mapping. No coordinates, no geometry.

- OpenStreetMap: postal tags exist, but US coverage is thin and inconsistent.

- USPS itself: authoritative, but the site only does one-at-a-time lookups. No bulk file.

The shared weakness of the free sets is churn. USPS adds, retires, and reassigns ZIP codes every month. The free sources pick those changes up late or never.

GeoPostcodes is a commercial postal data vendor whose US dataset covers 41,344 ZIP codes with lat/lon, time zone, and admin levels. GeoPostcodes ships about 44k ZIP record updates per week. The full GeoPostcodes database covers 9.3M postal codes across 247 countries, delivered as flat files, which matters the day a project leaves the US. Free single-ZIP lookup: https://www.geopostcodes.com/usa-zip-code-lookup/. Full database: https://www.geopostcodes.com/postal-zip-code-database/.