Windows Users: "open up Explorer and type size:>500MB into the search bar. You can tweak this to be anything you want, but I thought 500MB was a good starting size. Press Enter and it'll show you the biggest files first."
OS X Users: "open up Finder and go to New > Smart Folder. Click the plus sign on the right side of the toolbar. Click the "Kind" dropdown and go to Other > File Size. Then, enter your file size in the box, choose "Greater Than" from the dropdown menu, and you should see all the big files on your computer. Click Save to save your smart folder in the Finder's sidebar for quick access later."
Easy, right? As a Mac user, I find some of the easiest to find duplicates to delete are in your iTunes music folder. Also removing video files you've already watched and realize will never watch again make for obvious removal. Other easily forgotten files are ones attached to applications and games you've long deleted, so do a view in your Downloads and Preferences folders by file size and make sure you're not harboring unnecessary files just taking up space.
Image: Gregory Han/How Many External Drives Do You Own?