I usually use something like du -s * to find used disk space, but this utility is pretty handy.
http://www.chiark.greenend.org.uk/~sgtatham/agedu/

Just run the following commands and it provides you with a web interface you can drill down to to see used disk space and how old the files are:

agedu -f /tmp/agedu.dat --scan /
agedu --address --auth none --web -f /tmp/agedu.dat