There are a number of free online services useful for site builders. The four listed below are helpful for checking if DNS changes have been picked up, troubleshooting SSL errors, finding the cause of a site slowdown for speed optimization, and using search engine auto-suggest features to gain insight into common searches.
What's My DNS
DNS changes typically take anywhere from a few minutes to 24 hours before they make their way around the world and you can count on the changes being live. Wouldn't it be nice, when making DNS changes, to know when they had propagated? What's My DNS Propagation Checker does just that. Type a domain name into the search box and see what IP address 21 DNS servers from around the world are currently listing.
Check for SSL Issues
Running your website on HTTPS is almost mandatory now. Web browsers are showing sites not using SSL as being a security risk. If you set up a site on SSL or are switching a site over, the site Why No Padlock - SSL Check can tell you if everything has been configured correctly or, if not, what needs to be fixed.
Check Site Speed
No one likes to visit a slow site and Google penalizes websites that take too long to load. The Pingdom speed check service tells you the load time and is a good check to perform when optimizing your site.
Article and Headline Ideas
Answer the Public uses the auto suggest features of Google and Bing to show you the types of questions people are asking. It is a good resource for researching content creation ideas.
That's my list of frequently used services. I hope you find something useful. Have I missed a good one? Please leave me a comment and let me know.