iStat is an advanced Mac system monitor for your menubar.

DaisyDisk is a disk space analyzer for OS X.

Stellar SpeedUp Mac is a set of utilities to deliver high-end performance on Mac OS X.

Stellar Drive Defrag is a drive defragmentaion utility.

JDiskReport enables you to understand how much space the files and directories consume on your disk drives, and it helps you find obsolete files and folders.

GrandPerspective is open source software for Mac OS X used for disk space analysis.

DiskWave is a free disk usage utility for Mac OS X.