I saw this first over at Lifehacker.

Omni DiskSweeper is a small application for OS X (yes, Snow Leopard safe) that shows you a list of the hard drives and or network drives that are connected to your Mac, allows you to view the size of them, and then allows you to delete what you will.

When the software is first opened you are greeted by all of the drives that are available to you.  When you click on a drive you are presented with folders and files on that drive. Next to the folders and files is the disk space that they are eating up.  System folders and files are differentiated in red.

Basically you can look for files and folders by size and decide if you want to get rid of them.  I cleared out about 36 GB of space from my new MacBook, mostly because I am a Mac noob and never cleared out my .trash folder.  There were a ton of applications that I have tried and deleted, movies that I have burned, music, etc.

Mentioning this software is due only to the shear fact that it saved me about 36 GB of space and that it is so easy to use.  The application is free and available here.