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Extracted list from <http://www.takecontrolbooks.com/maintaining-mac>:
Oliver likes this one, which is commercial-quality, but still free:
- OmniDiskSweeper: http://www.omnigroup.com/products/omnidisksweeper/ (free)
Other options, with notes from Oliver:
- DaisyDisk: http://www.daisydiskapp.com/ ($9.95)
- Disk Inventory X: http://www.derlien.com/ (free) Note date of last refresh: 2005-10-09, as of March 2013. BUT, works great with 10.8 iMac. Shows folder size, too.
- GrandPerspective: http://grandperspectiv.sourceforge.net/ (free) See http://grandperspectiv.sourceforge.net/HelpDocumentation/GrandPerspectiveHelp.html. Required me reduce10.8's default security setting in order to launch application (since not signed). Didn't show folder size.
- JDiskReport: http://jgoodies.com/freeware/jdiskreport/ (free) Downloaded file reported as broken when tried on 10.8 iMac, March 2013.OmniDiskSweeper: http://www.omnigroup.com/products/omnidisksweeper/ (free)
- WhatSize: http://id-design.com/software/whatsize/ ($12.99)
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