Stephen, that was the first link I found using a Google lookup. 
I don't want to do that, I want to check the availability of different extensions.
This question is really about presenting server-side results that require lookups to other servers in a quick and user-friendly way.
I changed the topic title, because this does apply to more than whois lookups.
Moreover, ajaxwhois.com doesn't present any source code or sample scripts, and no articles about creating such a thing yourself, it's only a showcase of what's possible...
So if you know of a good code sample or article, please post it here!
Thanks!