Automatic Numbering with CSS Counters
The CSS 2.1 spec makes inserting counters into content possible. This has always been possible with ordered lists, but this counter system is much more robust allowing you to insert counters on any type of page element, set up incrementation rules, resetting rules, and more. The Opera Dev Blog has a nice article on how to do it. Works in current versions of all the major good browsers, and is slated to work in IE 8.
Typographic Marks Unknown
Familiar to most, yet understood by few. Things like the Pilcrow ( ¶ ) and the Section Sign ( § ).
Embarrassing CSS Mistakes
David Walsh goes all confessional and shares his Worst CSS Mistakes. Pretty funny stuff like setting font size with the star selector, then trying to make some text larger and having to fight against it with all nested elements.