Below are some useful web resources for web designers.

The W3C Validator tool validates the html and CSS code on a webpage and flags any errors found.

The Invisible Text Tool flags text that is either too small or similar in color to the background color to be readable by human visitors.

This site offers a free Web Page Analyzer tool. This Free web page analysis tool calculates page size, composition, and download time. Gives speed recommendations based on best practices for usability, HCI, and website optimization. It displays the total download time and size of all files on a web page and detects whether mod_gzip or mod_deflate has been enabled in the .htaccess file on Apache systems.

If you don't know how to turn on mod_gzip (Apache 1.x) or mod_deflate (Apache 2.x) in your .htaccess file residing in your root directory on your server, I recommend www.samaxes.com for mod_gzip and Nadeau Software Consulting Website Optimization for mod_deflate and mod_gzip scripts. You simply cut and paste the code into your .htaccess file and upload it to your web server.  This assumes, of course, that your web hosting company has enabled this feature and that your hosting service uses an Apache server.
The Duplicate Content Checker will identify and flag duplicate content it finds on a webpage.

The Bad Neighborhood Checker identifies outbound links that may be flagged as spam sites by major search engines.

The SEO Keyword Suggestion Tool is a good tool to identify popular keyword phrases that customers would use to find your product, service, idea or topic.

Keyword Density is a testing tool that measures the number of times a a single word or phrase appears on a web page. A density between 0.65 to 2.5 percent is considered to be a good range.

HTML Document Type from Microsoft is explained here.

Keyword Optimization is a good resource for testing the keyword optimization on a web page.

Page Rank Checker is a free web resource to measure the importance of a website on the world wide web. Up to twenty URLs may be checked at a time.

Google Places allows you to list your business for free.

My IP Address returns your IP Address when provided with the domain name.

Automatically Generate a CSS Sprite File with this FREE tool.

IE Tester - Free S/W Download is a good resource to test Internet Explorer Browsers ranging from IE-5.5 to IE-9 releases.

Here is a free web tool for checking the download time of a web page.

Here is a free windows keyword research tool.

Browser Shots is a good resource for testing all types of browsers. Free to use. Jobs are submitted as batch jobs.

Page Rank by Data Center is a good resource to measure the ranking of keywords by data center.