For an open-source equivalent on Unix-like systems, xdiskusage is good. (I like the xdiskusage graphics style better, too. It's very plain, and easier for me to understand.)
One handy feature of xdiskusage is that you can supply the output of "du" into it separately. This might be handy if you want to capture periodic snapshots of your disk usage allocation: just save the output of du each month or so, and run xdiskusage against some old snapshot whenever you need to. Or one trick I do often is SSH into a headless system to analyze its disk usage:
ssh some-server du -x / | xdiskusage -
For Linux: xdiskusage. Available on all major distros. By far the cleanest, simplest, easiest-to-read display I've ever seen. I've often wished there was something that useful for Macs.