These are just a few of the sites that I have found extremely useful in my own work. It's something of a personal selection and a bit haphazard but, hopefully, you will find something useful here too!
This is a site that allows you to check any domain name or IP address and find out to whom it is registered or linked.
The World Wide Web Consortium is an international community that develops open standards to ensure the long-term growth of the web.
This is a service provided by W3C that allows you to check the validity of web markup languages. These include HTML, XHTML and cascading stylesheets amongst others. You can also use this service to find broken website links.
This site will index up to five hundred pages of any site for free, and produce an XML sitemap that you can use on your site for search engine crawlers to use.
This is a site where people ask questions about a whole host of IT systems and facilities, from operating systems and protocols through to specific applications. Leo Notenboom worked for many years as a Microsoft software engineer but is a very approachable guy with plenty of commonsense comments and some good advice. If you have a problem, it's worth checking out his site...