How do you view file sizes in Leopard?

As the title states, how do you see what exactly is on your HD? I am curious to find file sizes and location. I have an 80GB HDD and have few to any extra programs installed. No music, no videos, etc. However, I am only showing 38GB available. There must be some huge files hidden somewhere, and I would love to delete them. Is there something built into Leopard, or can I download something from the net to help out? Any help is appreciated.

As for viewing file sizes...

Control-Click (right click) the file in the Finder. Select 'Get Info'.
or
Command-I

For a more dynamic option, try Option-Command-I. When you select different files, the info window will update to the currently selected file.

You could try Disk Inventory X or GrandPerspective.

You can also use SPACE. Just mark the file, and hit the space button and you'll get the quick view info

Thanks for all the helpful replies !! This is a great forum.