W3C Validator
validator.w3.org
W3C Validator is a very useful tool for checking the coding on your website. You can either upload a web document (html, xhtml, dhtml etc) to be checked or you can point the validator to a url and have it checked.
Your code will be checked against the coding standard you have specified at the start of the document and any errors will be listed after validation is complete. This tool is important as valide code is needed for cross browser compatability and also a valide page will be more respected by Google than one with errors in it.
There are also other tools available such as a Link Checker (for broken links), CSS Validator and many more.
Unfortunately W3C Validator does not have a batch file checker but if you need to check multiple pages there is a batch tool available from Htmlhelp.com.