Before I switched to a Mac, WinDirStat was my favorite tool for figuring out what was taking up space on my hard drive(s).
There doesn’t seem to be a great free alternative on OS X, but it’s been totally worth it to spend $12.99 on WhatSizeMac (also available on the Mac App Store).
When you first run it, you get a list of your drives. Click on the one you want to analyze, and it will start spinning…
When it’s done, you’ll be in a folder-browsing interface that lets you drill down and find the files and folders that are taking up the most space. This has helped me clear out a lot of unneeded files from my hard drive (video footage that I didn’t need anymore mostly).
Check it out!