Lean Domain Search - Find a Great Domain Name w/ twitter handle availability
Domainr - Domainr is the only ICANN-accredited domain status API provider.
Panabee - Best way to search domain names, esp for apps and startups
Instant Domain Search - Search domain names instantly by showing results as you type.
Mashword - Find a name and register a domain for your business, baby, project, and more. Mashword is a powerful company and human name generator that determines the pronunciations of entered words and then creates unique combinations and variations.
BrandHunt V2 - Search/track domain & social media handles in real time.