Domainr - Domainr is the only ICANN-accredited domain status API provider.
Whoisology - Connected database of whois records and whois history
Lean Domain Search - Find a Great Domain Name w/ twitter handle availability
Bulk Whois API - Bulk Whois API is a provider of parsed WHOIS data in JSON with simple API, competitive pricing, top notch support, and more.
Instant Domain Search - Search domain names instantly by showing results as you type.
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.