W3C: Markup Validation Service
http://validator.w3.org/
Favelet:
http://tantek.com/favelets/
Firefox Plugin (Web Developer Extension):
http://chrispederick.com/work/web-developer/
CSS Optimizer:
http://www.cssoptimiser.com/
The Definitive Post on Gzipping your CSS:
http://www.fiftyfoureleven.com/weblog/web-development/css/the-definitive-css-gzip-method