It was part of a long thread among WordPress hackers over the summer and fall, but this post at VulgarisOverIP just reminded of it: minify promises to be an easy way to compress external CSS and JavaScript without adding extra steps to your develop/deploy process. No, really, look at the usage instructions. (To be clear, the Vulgaris and Google Code versions are different, one derived from the other and backported to PHP4 compatible. Still, the concept is the same.)
Vulgaris reports a nearly 300% decrease in time to download, definitely worth the effort.
