Domainr - Domainr is the only ICANN-accredited domain status API provider.
BrandHunt V2 - Search/track domain & social media handles in real time.
Lean Domain Search - Find a Great Domain Name w/ twitter handle availability
Domainz.io - Premium domains, for not so premium prices
Instant Domain Search - Search domain names instantly by showing results as you type.
Brandpa - Domain names for startups