Web diagnostic tools are useful for several reasons. They are useful because they reveal important information about websites such as Google PR (Page Ranking), other Ranks (Alexa, Technorati, Quantcast, Compete), Whois information, back links, number of pages indexed, SERP, and other important web related or domain data.

That sounds like a lot of information in a single sentence. In essence, such information can be broadly categorized into seven categories:

1) Basic website domain info

This information reveals when the domain is created, when it's going to expire, and how fast the server loads. You may also see when was the last time major crawlers (such as Googlebot or MSNbot) visited the website.

WebSite / Domain Ranking

This section reveals all the ranking information provided by various ranking sites such as Alexa. You will be able to see rankings such as Google page rank, traffic rankings, blog rankings and more.

Search Engines Result Position

This is where you will see the search engine rankings for major keywords. This is important because many people who are looking to buy sites want to know which keywords are bringing in the traffic, and the tool is able to reveal the information.

Major Internet Directories

This is another area where many webmasters are concerned about. This stems from knowing how actively a site has been promoted previously. For example, you will be able to see if a site is listed in major web directories such as DMOZ directory or Yahoo directory. Bear in mind that is is not easy to be accepted by these directories, and the links are deemed as valuable by the webmaster community.

Back Links and Outbound Links

This is useful because webmasters want to know how many authority sites (such as .gov or .edu) sites are linking back to the website. The more links from authority websites, the more valuable the website. In addition, you will also be able to see where the site is linking to. These are called outbound links. In general, the lesser the number of outbound links, the better.

Indexed on Major Search Engine

This section shows the number of indexed pages on the major search engines. Established sites that have been around a long time are usually deeply indexed by the search engines. That means higher traffic levels. Newer sites tend to have less pages indexed by the search engines.

WebSite / Domain Title - Description - Keywords

In the final section, you will be able to see information for search engine optimization purposes. Such information include domain title, meta keywords, and meta description. These are usually the keywords that the website is optimized for. The more popular the keywords, the more traffic the website is going to get.

Web diagnostic tools, although very useful, are often available free of charge. If you find the tools useful, you may wish to support the tools by donating a token sum of money. This is a nice gesture to let the developers know that you like their tools. After all, these tools did let you check the status of your websites, so that you know where they stand.