DaisyDisk is a graphical utility for the Mac that allows you to see what's using up the storage on your computer.
I've used a free tool for a while called GrandPerspective which certainly works - but it's nowhere near as nice looking or as nice to use as DaisyDisk.
DaisyDisk is great for finding out where all the space is going on your hard drive. For example, I had no idea that I had 60+ gigabytes in podcasts stored on my computer that I didn't need to keep around.
These kind of apps are especially useful as more and more people are switching their primary storage to something smaller instead of larger. With SSD drives becoming the norm in laptops like Apple's MacBook Air, you need to make sure you're doing a better job of managing all the data on your computer - at least if you're a digital pack rat like I am.