Let's say you wanted to have a domain availability checker on your site. How would you go about setting one up? Is there some sort of master database that can be searched? Do you have to pay to have access to it? Pay per query? What about whois?

You need to use an API of a registrar that supports one

I'd go with http://whois.bw.org or MWhois (www.mattsscripts.co.uk (http://www.mattsscripts.co.uk))

I would go to http://hotscripts.com and search for 'whois'. Lot's of stuf!

Let's say you wanted to have a domain availability checker on your site. How would you go about setting one up? Is there some sort of master database that can be searched? Do you have to pay to have access to it? Pay per query? What about whois?

no it's free.

although some TLDs are quite hard to check. for example if you want to see the owner(registrant) details of a .EU domain than you have to go to www.whois.eu (http://www.whois.eu) . Doing a whois check with a script only tells you that it is registered and at which registrar or ISP.

mattsscripts - i mentioned before - is a very very easy php script to do whois look ups, you just need to upload it and that's it...

I usually use crucialwebhost and godaddy to check domain availability.