W3C compliance tools

W3C Markup Validation Service: http://validator.w3.org/
W3C Link Checker: http://validator.w3.org/checklink
W3C CSS Validation Service: http://jigsaw.w3.org/css-validator/

All W3C Validation links:
http://www.w3.org/QA/Tools/
http://www.anybrowser.org/campaign/abdesign3.html#validation
http://watson.addy.com/
http://webdesign.about.com/od/htmlvalidators/a/aa092799.htm

Browser Compliance
Acid1 Test: http://acid1.acidtests.org/
Acid2 Test: http://acid2.acidtests.org/
Acid3 Test: http://acid3.acidtests.org/