Melissa is a data quality company whose free online lookups and paid APIs verify addresses, append ZIP+4 and return details about a ZIP code. Searches that pair Melissa with ZIP code usually want one of those lookups. ZIPCodeLookup.io is independent and not affiliated with Melissa.
Address-level tools
Melissa's online lookups include address verification with ZIP+4, ZIP-to-city lists and other single-record checks, with daily limits for free use and APIs for volume. These are the right tools when you need a verified delivery address rather than a place record.
What a reference directory does instead
This site answers the five-digit question quickly: recorded place, county association, map reference point, nearby ZIP codes and the full list for each state, with no sign-up. It does not verify addresses or return ZIP+4; for that, use the USPS lookup or a verification provider. See ZIP+4 lookup.
Integrating either one
The ZIP code lookup API guide explains how to choose between a reference dataset and a verification API, and how to design honest responses in your own application.