Semi-transparent backgrounds are nice. They would be more popular, but Internet Explorer doesn't support .png transparency. There are a few clunky workarounds. Here's another that's a little less clunky.
Update: Here is probably the easiest way. You can stop reading this page now.
Update: The kind folks on css-discuss point out some other methods. I believe most of them use javascript.
Update: An article from A List Apart has more techniques (some of which are downright painful to look at...)
Update: Workaround for link bug in IE 6.0
Update: Now works in IE 5.0 (Thanks MG)
Example:
I like flowers.