Note: For those of you interested, as of May 2014, my database has grown to over 100 million domain names. I am now offering this domain list for purchase.
A reverse IP domain check takes a domain name or IP address pointing to a web server and searches for other sites known to be hosted on that same web server. Data is gathered from search engine results, which are not guaranteed to be complete. IP-Address.org provides interesting visual reverse IP lookup tool. Knowing the other web sites hosted on a web server is important from both an SEO and web filtering perspective, particularly for those on shared web hosting plans.
More about this tool. Set an API Key.