Here are a collection of free website resources. This list is angled at helping you run a website more efficiently. Please add more sites in the Comments and we will migrate them onto the page, if suitable.
The Best Internet Network Information Site
There are too many tools to describe here, but if you need to know anything about a web site, URL, email address or web routing then this is the place to go. The full toolset is now only available by paid subscription, but there are various free tools and trials here. This is the site the pros use.
Scrutinize Your Web Site
This site brings together on one page, a whole range of web site validation tools.
This is the #1 site for web developers as it tells you if your code is correct or not.
This site has good background, reviews and tutorials on website content management systems (WCMS). Also some of the best resources available on search penalties / website bans.
Free Service Validates Web Pages
Most web validation sites tend to either concentrate on syntactical validation or standards compliance. This site, which will check individual pages or whole sites, checks both. If all this sounds too tedious try the second link below.
What Your Web Site Really Looks Like to the User?
Every web developer curses about the interpretation and rendering differences of the different brwosers out there. What looks great in Firefox might be a total mess in IE. Oper, Safari and other might display your site differently too. It is a good habit to check what your site looks like in all these browsers. This Brwosershots.org offers such a service. Just enter the URL and you will be able to access screenshots of more than 70 browsers and broswer versions.
What Your Web Site Really Looks Like to Search Engines?
Here you'll find an excellent free tool that shows you how your web site appears to a search engine crawler. Using the tool to browse some familiar sites is an educational experience. For example sites that enclose all their content in frames appear as totally blank. Yes, folks that's right; most search engine crawlers can't see inside frames. Don't take it too hard - at least you now know why your site has never been indexed ;>)
Free Web Page Content Monitoring
This is a free service that lets you know when the content of a web page you have designated has changed. There are a number of sites that offer this service but what impressed me was the fact that the email you receive notifying you of a change, also details exactly what has changed. This means that you don't have to visit the monitored site as all the information you want is in the email. This gets around a real problem with these services; you get notified of a change only to visit the site and discover the change is trivial.
Everything You Wanted to Know About Cookies
Well not quite everything. This site covers the basics pretty well, has an excellent FAQ and gives good guidance on cookie removal but there's not much on the curly issues such as setting cookie management policies.
Free HTML Tutorials
This site offers an excellent directory of HTML resources including many free tutorials. While at the site, check out their free HTML editor. I've not tried it but, based on the description, it sounds most impressive. If you try it, give me some feedback and I'll pass it on to other readers.
Who Are You and Where Are You?
Yougetsignal.com offers very useful and interesting information about your connection and other helpul internet data. Your IP address, your Internet hub location, visual trace routes, Whois lookup, and, and, and. The list of tools seems endless. Definitely worth a check...