Smarty - Smarty provides address validation, autocomplete, geocoding and reverse geocoding services covering addresses in over 240+ countries.
Loqate - The Standard in Global Address Verification
Addressy - Addressy is address verification software that captures and verifies addresses on websites.
Melissa Data Quality - Melissa helps companies to harness Big Data, legacy data, and people data (names, addresses, phone numbers, and emails).
Experian Address Verification API - Experian provides Address Verification API for accurate, complete USPS addresses with minimal data entry.
AddressZen - Effortless Address Lookup.