1. Check website compliance to W3C standard:

The “Worldwide Web Consortium” (W3C) is an international consortium to develop web standards.

Its mission is: “To lead the World Wide Web to its full potential by developing protocols and guidelines that ensure long-term growth for the Web.” (www.w3.org)

- Go to http://validator.w3.org and test all your pages to identify any programming mistake that makes your website not compliant to W3C standard.

- If you use CSS, then use a CSS validator at http://jigsaw.w3.org/css-validator/validator-uri.html

- To check the syntax of Atom or RSS feeds, you can use: http://validator.w3.org/feed

- To verify if you have broken link on a page, use: http://validator.w3.org/checklink

2. Tools to see your own site as Googlebot (=Robot used by Google to read your site)

Sometimes you check if your code, your backlinks and architecture are good but you can't identify what is the issue on your site. One option is to visit your site using the user-agent: Googlebot. You will see your website and pages with the same eyes as Google.

- User-agent switcher: it is a Firefox extension. It "adds a menu and a toolbar button to switch the user agent of the browser. It is designed for Firefox, Flock  and Seamonkey, and will run on any platform these browsers support, including Windows, Mac OS  X and Linux".  It should be enhanced with the User-agent list.

- Detect Headers: This tool helps to detect the exact headers status codes that a specific URL generates.

3. Tools to see your own site as a spider

Here are two similar online free tools:

- http://www.webconfs.com/search-engine-spider-simulator.php

- http://www.webmaster-toolkit.com/search-engine-simulator.shtml