Though not fully supported in all browsers, CSS3 pseudo class selectors are really powerful and exciting. I’ve been using the basic CSS pseudo classes (e.g., a:hover) for ages, but I’ve only recently started using other pseudo classes, particularly the position/number-based ones.

One example is :nth-child(N) which allows you to style every other item in a list…

li {
color:blue;
}

li:nth-child(2n+1) {
color:red;
}

…or every third item…

li {
color:blue;
}

li:nth-child(3n) {
color:red;
}

…or just the fifth item…

li {
color:blue;
}

li:nth-child(5) {
color:red;
}

This tester from CSS-tricks.com demonstrates how it works. They also have this excellent explanation of nth-child and advanced pseudo classes in general.