Personally, I am not an SEO professiional, simply because until today I am still learning.
But what I have learn from many professional website developers and SEO professionals is when you are developing or creating a website, you need two things.
First, is personal skill. This is what don’t get instantly. I believe you will agree with me that to become a master on a specific subject it takes time, process and great effort.
Second, is the right tools, this is what you can get instantly.

If you are using Mozilla Firefox as your web browser, there are many free extensions that you can use.
Below are some useful Firefox extensions for developing professional website :

HTML Validator (http://users.skynet.be/mgueury/mozilla/)
Validates website pages to the W3C HTML standards with a simple green signs in the corner of the page if the page validates, a red check if it doesn’t validates and a yellow exclamation if there are warnings. It also includes an enhanced view of source code that allows you to determine where the faults are within the computer code.

FireFTP (https://addons.mozilla.org/firefox/684/)
Outstanding, safe, cross-platform FTP client that provides easy and intuitive access to FTP servers. This gets rid of a piece of application for those of you who utilize a separate program for FTP.

Professor X (https://addons.mozilla.org/firefox/2823/)
Grants you view header data without having to see source code. The page “scrolls” down and Professor X presents you the contents of the page’s head element, including Meta, Script and Style content.

NikkelWHOIS (https://addons.mozilla.org/firefox/2646/)
Watch the WHOIS info for any page by clicking the button on the top-right of the browser.

IE Tab (http://ietab.mozdev.org)
Tired of swapping between Internet Explorer and Firefox when testing out a web page you’re developing ? Utilizing IE Tab you can watch Internet Explorer in a Firefox Tab.

FireBug (https://addons.mozilla.org/firefox/1843/)
An advanced debugger console that grants you monitor on your JavaScript, CSS, HTML and Ajax.

Codetech (https://addons.mozilla.org/firefox/1002/)
Web page editor that has the taste of Dreamweaver.
An spectacular extension for anyone making web design that doesn’t want to spend out a few hundred dollars for Dreamweaver.

Server Switcher (https://addons.mozilla.org/firefox/2409/)
Easily change between websites on your development and live servers by clicking the switch server icon.

SEO for Firefox (http://tools.seobook.com/firefox/seo-for-firefox.html)
Pulling useful market research data right into Google’s and Yahoo!’s search results, including Google PR, Age, Backlinks, Alexa rank, WHOIS, and many more. It also adds a few helpful links on the top of the search pages, including Google Trends, Google Traffic Estimator, and the Overture View Bid tool.

Yet Another Window Resizer (https://addons.mozilla.org/firefox/2498/)
Allows you to resize browser windowpane to default screen resolutions.

Screen grab (https://addons.mozilla.org/firefox/1146/)
Gets a screenshot of the web pages and delivers it as an picture data file. This saves up time compared to the method I used, take a screenshot and open Adobe Photoshop to crop the picture.

Server Spy (https://addons.mozilla.org/firefox/2036/)
Shows what type of HTTP server (Apache, IIS, etc.) runs on the visited site on the lower-right side of the web browser.

Related posts:

1. Computer and Internet Glossary
2. Cleaning Up Browser Cache
3. Introduction to CHM Files