From The Omni Group:
OmniDiskSweeper is a great tool for making space on Mac OS X, Mac OS X Server and Mac OS 9 disks. When you start OmniDiskSweeper, it presents you with a list of disks attached to your machine. You double-click on one, and a new window opens with a browser listing every file and directory on that disk, sorted by size. This is very fast for HFS+ partitions: it takes about seven seconds to process one gigabyte. UFS partitions are significantly slower. Then you browse through the files and directories, and delete the large ones that you aren't using any more. The free space on the disk and the ordering of the directories are automatically recalculated. If you aren't sure what's in a file, you can double-click it or drag it onto an app on it to open it. It's that simple.