If you're concerned about large files taking up your hard drive space, there are a pair of great free apps that let you look at your hard drive from the perspective of file size.

From their website:
What Size is a simple tool that allows the user to quickly measure the size in bytes of a given folder and all subfolders and files within it. You would be surprised at how many useless files might be laying around on your hard disks. The files and folders are automatically sorted by size, with the biggest sizes first.

And
From their website:
GrandPerspective is a small utility application for Mac OS X that graphically shows the disk usage within a file system. It can help you to manage your disk, as you can easily spot which files and folders take up the most space. It uses a so called tree map for visualization. Each file is shown as a rectangle with an area proportional to the file's size. Files in the same folder appear together, but their placement is otherwise arbitrary.

When I got them both, I expected that I would use Whatsize more often and pass up on the more abstract GrandPerspective. The opposite has come to be true. Whenever I wonder where all my gigabytes have gone, I fire up GP and find them right away. More often than not, there are some very large files that are no longer needed.