I run XCache on the server that powers this site. XCache is cool. Out of the box, it allows caching of PHP compile code in memory, after all once a file is compile you shouldn't keep needing to compile it on every page load, should you?
A while ago we started having issues running a website. Bandwidth. Upon investigation, we saw that the majority of bandwidth was being use by image files and of those, lot was from the ImageCache folder. It was also causing a lot of excess server load by consuming Apache processes. Something needed to be done.