Disk Inventory X was kind of nice, but it is painfully slow.
Are there any modern and faster alternatives?
I only care about the disk space, and not about the file type stats and about the graph which Disk Inventory X displays.
OmniGroup offers a free utility called OmniDisksweeper which lists files/folders sorted by size.
While modern and fast are not exactly the strength of a Java application, I can recommend JDiskReport. It's free and does only one job, displaying the sizes of directories on your hard disk.
However, it seems that the Mac OS X download is currently broken on Mountain Lion because of Gatekeeper, so I recommend the Linux, Solaris, Java enabled OS download and running the JAR directly, which works just a fine.