The DomainTools Whois plugin will automatically link domain names in a WordPress page or blog post to the DomainTools Whois service. It also generates a pop-up containing a summary of Whois information that your visitors can see without leaving your site.

DomainTools Whois Plugin

DomainTools whois plugin shows whois data for domains found in blog articles and links them to whois.domaintools.com.

cavimaster

Simple whois tool in a widget
Post a domain name et submit. 
Whois on widget return to you a complet whois query into Jquery lightbox. 
You can use shortcode like: [WHOIS] for put it in your page or post.
<a href="http://www.devsector.ch/cavimaster/category/developpement/wordpress-plugins-widgets" rel="nofollow ugc">Cavimaster_blog</a>
Original PHP whois code by Sven Cailteux – <a href="http://www.dimension-internet.com" rel="nofollow ugc">Dimension internet</a>

DomainTools Whois Plugin vs Whois on widget