If you just build a website and sit back thinking 'alright, it's done' then you may be in for a few surprises. There are things you need to check to make sure of before launching a website (and when a website is already running). Websites, like cars, require tuneups from time to time. The sites listed here will help you perform 'website mechanics'
Validate your HTML and CSS with these tools.
W3C markup validator service
Validate your HTML.
W3C CSS validator service
Validate your CSS.
Check to see if you have broken links on your site with these tools. If broken links are found, fix them so that your visitors don't get the 404 error page.
W3C link checker service
Make sure you have no broken links on your site.
Xenu Link Sleuth
A very thorough link checking program. Download it.
Cross browser testing
See how your browser looks on different websites/screen resolutions/operating systems with these tools.
Excellent site where you can perform lots of testing to see how your website will look in different circumstances.
Similar to Browser Shots. Has a free trial.
Will build a sitemap of your site for you.