Tools

I am a Chrome user. I had no use for the Firefox browser until I found these extensions.

Firebug: Chrome has good element inspection but there are just a few little additions that make Firebug better.

Web Developer toolbar: Great controls for testing

Site performance is a factor in users' perception of the quality of your site and in search engine ranking so do not ignore this aspect. There are several tools available to test the loading speed of your pages.

Pagespeed from google is my favorite. It is available as a browser extension or as an online service. Due to the announcement of the closing of Google Labs it may be going away.  Hopefully they will keep it going.

viewdns.info This site lets you access just about any aspect of DNS settings you might want to know about. Have your DNS changes propagated around the world? Can your site be accessed from China? What other sites on your shared hosting server?  Don't be caught not knowing the answers.