My absolute favorite tool. Ever. Firebug is a Firefox Extension that lets you view and manipulate html and stylesheets straight from your browser, in real time. And, while you cannot save you changes straight from Firebug, it remains an excellent tool for debugging CSS issues! This is a must have.

( Firebug Homepage )

2. Xenu

And no, I’m not referring to the one that flew a jet plane through outter space. Xenu is an easy to use application that will very quickly allow you to identify any broken links, or missing files on your site. It’s very lightweight, and gets the job done very quickly. All you have to do is specify your web site’s URL, and voila- complete analysis. Always use XENU before and right after you go live with a website.

( Xenu Homepage )

3. Google

That’s right- Google. Let me explain. Google allows you to view all of your indexed pages simply by searching for site:example.com. How’s that useful? 301′s! Whenever you perform a major site revamp or update, it is imperative that you 301 the old pages to their new counter parts. This helps maintain page rank. Yahoo supports the same command syntax. ( You know where to find google… )

4. W3C Validator

The W3C Validator is a nice little tool to have around. Having a website that validates is important, and will improve overall SEO. By how much remains debatable, however, it’s not something to just overlook foolhardily.

( W3C Validator )

5. Web Sniffer

While I don’t use this one often, it’s still very handy. With Web Sniffer you can easily test the headers ( and other useful information ) that your pages are returning. Particularly good for making sure your 404′s, 301′s, and 200′s are valid.

( Web Sniffer )