Domainr - Domainr is the only ICANN-accredited domain status API provider.
BrandHunt V2 - Search/track domain & social media handles in real time.
Instant Domain Search - Search domain names instantly by showing results as you type.
Lean Domain Search - Find a Great Domain Name w/ twitter handle availability
NameMing - Get smart domain name suggestions using synonyms, suffixes, prefixes, short url, new gtlds to find the perfect domain name. Generate ideas for social user names and check availability for free!
GoDaddy - GoDaddy makes registering Domain Names fast, simple, and affordable. Find out why so many business owners chose GoDaddy to be their Domain Name Registrar.