My favourite Firefox addon

Marc Gueury’s HTML Validator addon for Firefox is an incredibly useful tool for anyone wanting to write valid HTML (and that should be all of us!). The HTML Validator addon displays the number of errors on an HTML page in the status bar of your Firefox window, you can then simply click the HTML Validator icon to see detailed information about each error – including which line the error is on, and a basic suggestion on how to fix it.
Get it here.