Domainr - Domainr is the only ICANN-accredited domain status API provider.
Domainz.io - Premium domains, for not so premium prices
Lean Domain Search - Find a Great Domain Name w/ twitter handle availability
Newsy - Monetize your un-used domains and turn them into your own content aggregator with 0 code!
Instant Domain Search - Search domain names instantly by showing results as you type.
Tool.Domains - Search free to register expired domains