Whoisology - Connected database of whois records and whois history
Domainr - Domainr is the only ICANN-accredited domain status API provider.
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Instant Domain Search - Search domain names instantly by showing results as you type.
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.
Lean Domain Search - Find a Great Domain Name w/ twitter handle availability