Open FreeSpace and you'll immediately find out how much space is available on your hard disks.
The window shows the total space, how much space is being used, and how much space is unused. It not only shows this information in numbers, but it also has a sort of bar graph that shows the amount of use.
For more information, including on later updates, please read the documentation or visit the home page.
FreeSpace is a Carbon application. If you use Mac OS X, it will run natively. If you use Mac OS 8.5 or better, it will also work natively. I used this program on both Mac OS 9.0.4 and Mac OS X, and it worked fine on both.
FreeSpace is freeware, but if you can't live without it, you can register it for $9.