As the W3C is the official source of HTML and CSS specifications, I usually recommend people stick with their validators. I have sometimes found third-party validators to be inaccurate and/or less than reliable. The W3C's HTML Validator is at
http://validator.w3.org/ and their CSS validator is at
http://jigsaw.w3.org/css-validator/.
The Webdeveloper extension for Firefox relies on W3C's validators. From a design standpoint it is always best to rely on Firefox as the primary browser during web development and only use MSIE for final checking as MSIE has so many rendering bugs and is so premissive of coding errors. Another great web development extension for Firefox is "View Source Chart". This extension allows one to review the HTML source in a structured format, which can help to quickly find bad nesting or coding errors.