As if there’s an offline one to check whether a domain is available or not. 😉
Psychic Whois is it.
It’s the best domain availability checking tool I know so far. AJAX powered, it instantly checks whether a domain is available for registration after you have typed the primary part by listing all 6 popular TLDs: .com, .net, .org, .biz, .info and .us and the name’s availability for each of them. Not only that, it suggests keyword combinations based from what others have searched and queried.
If you need to search for 100s of domains at one time, bulk domains search tool is what you need.