CSS: 15 Tricks you should know

1. Absolute positioning inside a relative positioned element.
2. Z-Index and positioning
3. Margin Auto
4. Use Padding Carefully and Appropriately
5. Hiding text using text-indent
6. IE Double Float Margin Bugs
7. Using CSS to Fight Spam
8. PNG in IE 6 Fixes
9. CSS Cross Browser Transparency
10. Use CSS Image Sprites
11. Use Conditional Comments to support IE 6
12. CSS Specificity
13. Achieving a minimum height in all browsers
14. The * HTML hack
15. Sliding Doors Technique

Source: http://blog.themeforest.net/general/15-css-tricks-that-must-be-learned/