Ask HN: What are some well-designed domain name checking tools?

Yesterday I read this post from HN: http://semmyfun.blogspot.com/2010/09/get-that-4-letter-domain-youve-always.html

And in the comments somebody mentioned two sites that check the availability of domain names. However, the author has since removed that comment, because he himself owns a site that provides a similar service (but is down for me right now). Does anybody remember what the two linked sites were, or can somebody point me in the direction of some well-designed and comprehensive domain name checking sites?