This is what my drive looks like after getting rid of the biggest space offenders:

The big gray block on the left (highlighted in white) is the post cleaning free space I have gained (approx. 65GB freed).
Get all the info here!
UPDATE:
I am not a MAC user but was made aware of a similar program for OSX called Disk Inventory X. You can find out about it here!
Hat tip: Lukas Blakk.
And of course we can't leave the Linux community behind... so if you are using a Linux distribution you can use a similar application called KDirStat, find out about it here!
Hat tip: James Boston.