DaisyDisk also gives you a great overview of all connected disks, be it Macintosh HD, Thunderbolt disk, flash, network storage, you name it.
All displayed data are updated in real time, so you can always tell if there's enough space to install a new game or download an HD movie.
HOW IT WORKS: It's easy. Just scan a disk and see all of your files and folders as a visual interactive map. Find an unusually large file. Preview its content, and delete if you don't need it anymore.
Using a disk space analyser is a great idea. They are very useful for identifying large files or large folders of files (such as a dump of files from a memory stick that you quickly copied to across 3 years ago and since then has been sitting idle in Desktop -> Old Desktop -> Backup -> Untitled Folder -> Memory Stick or some such path).
I use GrandPerspective (free) but Daisy Disk looks like it would be easier to use.
Or OmniDiskSweeper is always free too. Not as pretty but does a similar job.
This should be standard functionality built into the OS. In the 90s when Norton Utilities was still selling well, this app may have a chance.