CSS along with JavaScript manipulating the DOM is what helps make all those cool client-side effects. I'll be adding more links here soon, but for now this is a good start.

• sizzle.js (Sizzle JavaScript Selector Library)
• SitePoint.com CSS Reference
• CSS Positioning Properties At-A-Glance Guide [CSS Tutorials] 
• 7 Principles Of Clean And Optimized CSS Code
• QuirksMode (A greate site for browser compatibility issues CSS and JavaScript)
• Still don't think CSS can help you? Check these two crazy sites:
  • Ned Batchelder: CSS Homer, animated
  • Román Cortés >> Bush CSS
• Acid Tests
  • What is the Acid 2 test?
  • Take the Acid 2 Test
  • Take the Acid 3 Test
• Preparing for IE 8
  • The Internet Explorer Blog
  • Some articles on IE 8:
    • IE 8 strict mode doesn't allow for CSS opacity?
    • IE8 Version Targeting: Microsoft Considers Feedback
    • Gomez Announces Web Performance Testing Support for Microsoft Internet Explorer 8 | Reuters 
  • Internet Explorer Compatibility (7 and 8)
• 50 Really Useful CSS Tools (Thanks codeproject.com for putting this in your newsletter)