Reasons for health checking a website:

Quality Control:

To check that your web designer or supplier has delivered a quality product for you or to check that you've done a good enough job on a DIY site.

Developer evaluation:

To check out the work of a web designer or developer that you are thinking of using

Requirements / Improvements specification:

To put together a list of improvements that you can take to a web development company.

Underperformance diagnosis

You might find that you are getting traffic to your site but are not getting the sales conversions anticipated because of some fundamental flaw in your site's design, for instance you might be losing custom because your site takes too long to load or your site contains broken links.

Health checking tools.

Speed

People are impatient, nobody wants to wait for a site to load up - we all know how frustrating it can be waiting for a website to load can be - we don't want to wait: we want it now!

Web Page Analyser - 0.961 - from Website Optimisation

Free web site speed test. Calculates page size, composition, and download time. The script calculates the size of individual elements and sums up each type of web page component. Based on these page characteristics the script then offers advice on how to improve page load time. The script incorporates best practices from HCI research and web site optimization techniques into its recommendations.

Code Validity

Is the site coded using up to date and standard practices? Does it validate against W3C standards?

W3C Mark-up Validation Service

A free service that checks Web documents in formats like HTML and XHTML for conformance to W3C Recommendations and other standards.

Accessibility

Is the site accessible to a maximum number of visitors both able-bodied and disabled - you don't want to turn away any potential customers just because your site doesn't conform to some simple accessibility guidelines.

TAW

TAW (Web Accessibility Test) is a tool for the analysis of Web sites, based on the W3C - Web Content Accessibility Guidelines 1.0 (WCAG 1.0)
Analyses a site and runs automatic checks against Priorities 1, 2 and 3 of the WCAG.

WEBXact

WebXACT is a free online service that lets you test single pages of web content for quality, accessibility, and privacy issues.

Accessibility checker at Etre

Etre's Accessibility Check evaluates your page against subset of the WAI guidelines. These guidelines form the basis of most global legislation relating to accessibility.

Browser camp

Demonstrates how a website looks on a MacOSX browser. The free version simulates the Safari browser, the paid version simulates a full range.

Graybit

Tool designed to visually convert a full-colour web page into a grayscale rendition for the purpose of visually testing the page's perceived contrast.

Useability

Is the site "user friendly"? Is it task focussed and does it make those tasks easy to accomplish. Does it have a good range of features?Are basic site features such working & correct: do all the links on the site link to pages - and do they link to the right pages?

Silktide Sitescore Sitescore is the Swiss army knife of online tools which checks your site for range of features and rates how well designed, popular and accessible your website is. W3C Broken link checker Spiders through your site and checks for broken links. Doesn't however check for incorrently linked pages - this needs to be performed manually.

Findability

Is the site easy to find via search engines such as Google, MSN and Yahoo? For more tools and information about this area see the "Make Google love you website" event review. Page Strength at SEOMoz In SEOMoz's own words: "SEOmoz's Page Strength tool is intended to serve as an alternative to Google's Page Rank score in the toolbar, offering insight into how valuable, important and popular a site or page is as compared to others on the web.