InterNIC—Public Information Regarding Internet Domain Name Registration Services

http://www.internic.net/

There's only one good, official place to check to see if a domain name is available, and that is:
http://www.internic.net/whois.html

?Why?
Because, if you type a name directly into the address bar, and it comes back "404 --Not Found", that only tells you that it's not being hosted, it doesn't tell you whether someone already has the name, but NOT hosted.
AND, worse, I have heard instances (and unfortunately, this is all too common), when someone 'checks' a name at a third-party website, and then, before you know it, someone else has already snapped it up... apparently because the site is not secure, and unethical parties are watching it to see what names might be queried, and then buying and 'cyber-squatting', speculating that they can resell the names at a profit.
For more info on choosing a domain name, see the brief articles I wrote for my clients at :
http://danshaw.com/dan-shaw-writer.php
under "Website - Business Development", especially, "How To Make More Money on the Web."