Gibson Research Corporation – excellent website with a range of useful data from an expert in internet security.

w3 website validator – this is the official website validator for the World Wide Web Consortium (W3C), an international community that develops open standards to ensure the long-term growth of the Web.

ismysiteworking – a simple tool to assess the basics of your website.

The Spamhaus Project – excellent anti-spam resources.

websiteoptimization.com – Free website analysis tool giving a lot more detailed information.

my-ip-neighbors.com – my IP Neighbors lets you find out if any other web sites (“virtual hosts”) are hosted on a given web server.

alistapart.com – a List Apart explores the design, development, and meaning of web content, with a special focus on web standards and best practices – good to get more detail about website design.